I hate the look of the M6 personally. I say get the new M3 with the v8 that thing flys, and then with the cash left over it, get a nice ECU, get it tuned, lower it a bit, get some racing wheels, hit the autocross circuit and have fun. I personally love the look of the M3 I personally think it is the best looking BMW.
Don't get a Merc, take that thing into a corner like the M3 and that would be the end of that. Really Mercs are there to say look what I have something with power, but an automatic transmission... Get the M3 or if you really want the M5, and never look back.