WARNING: the following is based completely on my views as a player, and I recognize bias is ever an issue with opinion/observation-based articles. As such, take all that you read with a colossal size grain of that naturally occurring, flavor enhancing crystal.
The plasma gun boasts a solid S7 combined with AP2, granting the knowledge that no one is claiming a fancy armor save against this beast. The Rapid Fire type is a nice solid rounder which became much, much more valuable with the coming of 6th. Before hand it was more or less a worthless weapon type; these days, however, it is the bread and butter weapon type for a reason.
The grav weapons, while not complicated once you get to know them, are a mouthful of rules and wordings. It has S * which means it's given to you by a rule, and the same AP2. Again, its nice to know that what you wound isn't getting their armor save, but its rule does present some limitations on which targets are worth targeting.
In this respect the plasma gun wins out. It's simple, non-discriminant in it's targeting, and hosts some decent stats for dealing with just about any type of Infantry, most Monsterous Creatures, and even some Light Mech hunting. The grav weapon is capable of most of these, but sadly becomes a bit choosy when selecting less powerful targets, which can often be weak little troops holding a point.
It is fair to note that they both lose points for paying a hefty sum of 15 point when being fired on weak infantry. They both more or less want to hunt heavy infantry, but the plasma gun takes this category on sheer numbers.
Grav Tech: 5/10
Grav weapons have a unique rule that makes them a bit finicky when shooting at infantry, and pretty decent when firing at vehicles if you have the number of shots you need. In either case, tactical squads don't much care for either it's special rule or it's salvo weapon type as it does not gel well with their play style. However, the Gravitron rule is great when used with high volume of shots, and even without, the rule still offers some decent potential by itself.
Again... there is just no contest here. With both weapons, you're paying 15 points, and the Gets Hot rule just puts that all on the line with a risk of losing it with every shot you take. Gravitron rule puts some limitations on you, but offers some excellent rewards should it work out. While yes, plasma can utterly wreck a vehicle in one go, and nets the bonus on the vehicle damage table, that doesn't save it from a poor roll of a 1 turning into a 2, which is all the same. Against infantry it is less restrictive, but always that nag of "will my guy kill himself shooting" is ever present. So... while it loses points for some limitations, the Gravitron rule simply demolishes plasma's Get Hot rule.
Grav Tech: 7/10
Who has Access to Each?
|If you get the reference, well done.|
Next up is almost a bit unfair, but it must be considered. With the coming of the Devastator Centurions, there also came the Gravitron Cannon. This beast hosts an impressive Salvo 3/5 rule... and since Centurions are Slow and Purposeful, they always get the whopping 5 shots. It does beg the question why it's a salvo weapon when it can only be taken by an S&P unit... but perhaps we'll discover why down the road as GW continues to sell more supplements. This is tag-teamed with the Grav-Amp, allowing their shots to re-roll failed wounds and armor penetration results. We'll talk more on these numbers later, but trust me... it's more impressive than you might think. In any case, they have access to these potent Grav weapons, but there is no plasma.
Plasma loses out here, because the only unit that really wants it are tactical squads. Maybe combi-plasmas on Sternguard Veterans wouldn't be too bad... but I for one believe their spacial issue ammo is good enough. Beyond that there is little competition in most units, as they generally have the Relentless USR, which Salvo rule takes more advantage of than even Rapid Fire does. It's close, but grav tech just has more options, and some powerful uses given the nature of who can bring them.
Grav Tech: 8/10
Firing at Infantry
So how are those vicious centurions with their high volume of shots? Now... if you have a unit of three, not counting the missile launcher shots, you'll net an impressive 10 hits from grav weapons. If your target is an infantry or some non-vehicle unit, the numbers will vary based on their armor saves, but more or less you can count on somewhere in the neighborhood of about 7 to 9 wounds as long as you fire at heavy (3+ save or better) units. It falls off a bit when firing at low armor targets (5+ save or worse) with an average of 5-6... which still isn't bad by any means. But no matter how you slice it, Grav weapons are wasting potential when firing at poorly armored infantry.
Because neither is expressly bad, it's a slightly close call. However, given the extreme level of possibilities when it comes to firing at infantry, plasma's method of shooting first, and asking questions later is a much more consistent form of weapon design. This isn't to say it is better than grav at anti-heavy infantry, but it can be used against light as well with relatively similar results in either case. The biggest catch is that grav's use is limited against high model count, light infantry units which tend to look at you and yell...
The other catch is that generally speaking, grav weapons "need" units with Relentless or Slow and Purposeful to really make the best of their salvo type and Gravitron rule, limiting them primarily to use by bikes and devastator centurions; both of which the new codex really favors, but it is more or less another limitation to consider. So in the end, plasma gets the carrot here. It's important to note grav isn't expressly bad at anti-infantry... but it requires volume of shots to really shine.
Grav Tech: 6/10
Firing at Vehicles
Grav weapons on the other hand have a pretty solid means of dealing with mech. As long as you take what we've learned so far, and put these weapons on Relentless or S&P units, you can expect a decent chance at netting the magical 6 to flat immobilize your target. In most cases, prohibiting a box from moving will neuter it's use on the battlefield, and force its occupants out of their transport to remain relevant in later turns. This gives grav weapons a really solid place at ranged vehicle hunting. It also nets a unique opportunity via it's wording to prohibit the use of cover by mechanized targets (See A Closer Look: Part V for more on that).
Devastator Centurions specifically shine extremely well when you utilize them for anti-mech. Again, not counting the missile launcher shots, these 15 shots net you an impressive 1.89 6's, with math saying you should get the second and even a third in there after the re-rolls. Now considering what we've learned on how Gravitron works with the Vehicle Damage result "Immobilized" (Again see the above link to A Closer Look: Part V), you'll note that 2 hits spells doom for most vehicles in the game, and the third spells death for anything with five... yes five hull points or less. Even so, you apply the most powerful result on a vehicle damage table short of flat out destroying it.
At the end of the day grav weapons take this category. If not for their impressive numbers, but also by sheer fact that it comes without risk of blowing yourself up. However, I would be amiss in my consideration of these two weapon if I didn't mention one element which does favor plasma. With the high risk, also comes the chance of high reward. Because grav weapons can only ever inflict multiple hull point damage and immobilize the target, it is impossible to cause a vehicle to explode, where as plasma actually takes advantage of it's AP2 against mech, giving it a 1/3 chance at blowing the target off the table entirely. This explosion accomplishes a lot, namely more wounds it could cause from the explosion itself, as well as removing the hull from the table, giving the enemy no cover to break line of sight with as your shooting continues. Both plasma and grav weapons will cause the same pinning test from wrecking or destroying their targets, but plasma does have that in it's favor. It's worth the mention, but in this player's eyes, grav still takes the cake here.
Grav Tech: 8/10
Grav on the other hand is a solid choice in most situations... but it becomes more and more inefficient as you begin to fire on those weaker units. Even when firing at vehicles, you'll need a high volume of shots to get a reliable chance at landing a 6. It just has so many limitations that while they don't cripple it by any stretch, grav weapons have a specific few units it really wants to hit. It is very well suited for dealing with Biomancy Bugs, Land Raider Rush, and more or less heavily armored grunts, but outside of these examples it starts to fall of quickly. Additionally it has nay or few uses in tactical squads, given salvo rule.
While it's not a clear victor, plasma squeaks by with a narrow win here. Gets Hot can be very, very annoying, but you just can't argue with the impressive stats and high usability in most battlefield situations. You can find other things to shore up plasma weakness in your army list, but you can only shore up grav's weakness by bringing a lot of it. This can become a difficult task as the points for grav weapons begin to add up quickly, but if you do... they will serve pretty well in most situations.
Grav Tech: 6/10
Grav Tech: 40/60
So it's grav weaponry by a thin margin here folks. I for one am a fan of these new weapons. They aren't anything to write home about, but they certainly have much more usability than I feel most give them credit for. Not only are they great when matched with the units which the codex has introduced as being quite good (bikes and centurions), but they also offer a great weapon that cracks through heavy armor and mech without the pesky risk of killing yourself.
Beyond all of that, I could see an army that wishes to focus on the grav weapons doing pretty decent. The real winners with them are the devastator centurions. The sheer high volume mixed with re-rolls makes them fantastic at just about anything. The real point I walk away from after writing this is that Grav Weapons are fantastic, so long as you don't put them in the tactical squads. They just don't work well in there, as compared to plasma... or even meltas and flamers. Keep them on your centurions and your bikes, and you'll be pounding your enemies into the ground in no time... literally.
I hope this was eye opening for you. Not sure what my next Showdown comparison will be, but we'll have to see what time will bring us. Until then...