No Plasma

As I've said before, I have very bad luck when it comes to plasma weapons. Too many times. I've had a game, where every single plasma gunner I had, killed themselves from gets hot in the first couple turns of the game. Which came to 4 bikers, 4 plague marines, a total of 4 champions of chaos, and my lord on a bike took 2 wounds from a misfiring combi-plas. I was not happy.
Bad games aside, I also don't like running plasma because i feel they are the middle ground for special weapons. Flamers are template weapons that are excellent for taking hordes down.
Melta is the king of anti-tank guns, ignores every armor save and causes instant death on T4 or worse.
Granted, both of the above have very short ranges to be effective, but they are VERY effective at what they do, and they do it for cheap. They also have the advantage of being assault weapons so you can charge whatever your target was to finish it up or tie up that massive tarpit for the rest if the game.
Plasma is overcosted, can kill your own guys, and generally serves 1 purpose: killing terminators.
Okay so the first point I make can be disputed with combi weapons, but then whoeve paid for those combis aren't going to shoot them until at least 12 inches close to the enemy to make sure they get all their points worth. It just so happens that the plasma's effective range is the same as a melta guns max range. Both can nuke terminators the same way, and even better, melta has the benefit of instant death-ing those terminators if they're,
say, Paladins with an apothecary. No Feel no Pain, no sticking around for the next round.
Yes, plasma gets that extra shot and thus theoretically hits twice as often as a melta team.
Plasma also has the advantage of being able to pop most enemy transport options at a further range than melta, but is not as reliable at it. The way i think if that is that they're half range Autocannons that have a better chance of making the tank explode. But Autocannons always have that second shot, some are even twin linked, and shot density will beat out better explosion chance every time. After all: why penetrate a tank once when you can glance it to death for a lot cheaper.
Going over the gets hot rule, there is a 1 in 6 chance of wounding the gunner and then they have to make an armor save and a feel no pain save if they have one. That's a lot of checks to make sure your guy keeps firing away, but the one time he fails everything you will feel it. All the guys around him you bought as ablative wounds serve no purpose anymore. Or maybe your warlord takes his final wound off with his own pistol. Gets hot makes (at least me) you doubt your decision to fire. It throws doubt in your face because what if the gun beats the odds and kills your guy? He could've been that one wound you needed to keep that deathstar tied up in combat for another round, or maybe hes the laat member of his unit and he's valiantly holding that objective you need to push yourself to victory? Do you risk all that just to have a gun that may or may not the intended target? After all, whatever you shoot with plasma probably has an invulnerable save of some kind.
I just choose not to even play that particular Russian roulette anymore.
As for killing terminators, they are great at that: AP2 so they have to try their 5++, and being S7, they wound on 2s even if those terminators are marked with Nurgle. You can even shoot those terminators at 24 inches away just to keep them at bay another turn. But what if they're a paladin squad? If you shot 2 plasma guns at rapid fire range thats 4 shots, maybe 3 hits, about 2.5 wounds, which can get invulnerable saved and then FNP'ed away anyway. 2 melta guns gives you about 1.5 hits, a solid 1 wound, which bypasses armor and FNP severely dropping their survivability that they paid so much for.
At least in my army, dedicated anti tank is essential to give my artillery more targets to satirate, and melta gets the job done quickly and efficiently.
All that said, It comes down to preference: are you a cautious player that doesn't want to lose your expensive guns by accident? Or do you want a gun that can do everything to a degree and damn the possible consequences?
The choice is yours, this is simply my two cents worth.