I may have said this before in other threads, but I still think that gains are going to be very similar from most reputable MSS65 DME tuners.
Aside from the gains, I'd encourage you to call or contact all the top tuners out there and decide for yourself. With how advance these cars can get, I think it's rather crucial that you pick someone that you feel comfortable in contacting. Customer service, employee knowledge, and experience go a very long way.
I personally chose Evolve Automotive to tune my E90 M3 since I've only had amazing experiences working with them in setting up my E46 M3.
Best of luck with your decision! I'm sure you'll make a good choice regardless of what route you take.