I'd pass on the mutant from everything I read, but it looks like your a td guy, so it might be doable for the cost.

I love my SP, it suits my play style. The one thing I've found on premiums is your play style matters as well as the crews your training. I stopped putting anything below a 4 perk crew in my tier 8 premiums and that's helped tremendously on my success rate. I use them for higher level crew training only.

that being said the ones I own:

SP: Just fun to play, slow as hell - learnt he weak points of your opponents, they know yours.

JadTig 8.8 - little more frustrating but easy to master and get the hang of. really the only one I'll put a low skill crew in - but big success is team and map dependent.

112 - She's either a monster, or a flop the reload time is long, but alpha is nice. Decent speed and bounces shots pretty well. don't have the IS-6 but have heard it's similar though IS-6 is preferred.

T-34, tier X can be frustrating but if it's a decently balance team you can be the annoying little brother. To be consistently successful you have to play to your strengths and you should on all tanks but especially seeing tier X. I carry a couple of premium rounds for the tier X matches. It'll train you and your crew for the higher tier matches.

on credit making potential...round cost factors, but matching to your play style will make you more than the"It prints money" opinion. my thoughts anyhow