True commisars do always ruin fun things.
I'm running both these days. I run two squads of five Plague Marines with dual Plas and two squads of five Plague Marines in Rhinos with meltas. Plasma is great with FnP.
Yes i do have to agree that plague marines uses their plasmas wisely, but they will never beat exploding guardsmen with a commisar watching.
I only go Plasma for Company Command Sqauds due to having a medic for "feel no pain"(FNP).
5+ armor and 4+ FNP is way better to counter the "Gets Hot" rolls than carapace IMO.
I go Melta on the vets as it has a nice synergy with "demolitions" getting melta bombs for the whole squad. The demo charge is just a bonus.
The meltas are also assault weapons, so it works well with shotguns as a counter assault unit. Especially when some big nasty unit deep strikes/outflanks into your gunline.
One of the hardest things for me to get over was the 5pt upcharge on plasma, when they seem roughly equal. You trade off assault for rapid fire, melta for double shot, etc. Why 15 for plasma and 10 for melta?
My Plague Marines said good bye to plasmas half a year ago. My guys are offensive thinking, so I need assault weapons. And with meltas I can never go wrong (In larger games I have a squad or two with flamers as well).
If my style of play were to always sit back (on objectives) then plasmas probably would work.
Well, coming from a Marine standpoint, melta is pretty much the best period.
Why?
If we look at it in a different perspective, which one would be best on a combi-weapon? The melta, because a pair of plasma shots is worth maybe a dead Marine or two, whereas a melta to the back armor of a tank is worth a dead tank. So, applying the knowledge that melta is better than plasma on a turn-by-turn basis, it's pretty clear which one is the best. Granted, a plasma gun may have longer range, and thus get a couple of extra shots, but for necessity in 5th Ed for tank busting, and the improved AP1, makes melta my choice weapon. Behind flamers of course.
I'd say melta beats plasma 9 times out of 10, especially with the amount of mech you see, but plasma becomes more an more attractive the more you can mass in one place.
This is an extreme example, and I'm sure some mathhammer tourny champ will sneer but you could have 4 plasma guns in a command squad (!). You'll be reliably killing large armounts of pretty much anything but 2+ armour saves and probably at least glancing most transports out ther. Combine that with Feel No Pain and you don't really have to worry about Gets Hot.
Granted the further you're shooting, the more likely cover saves will ruin your fun, but thats no different from meltas at that point.