The SMG tranny is the same tranny as the 6 speed.
Only thing is the hydraulic components, but they are just like brakes. Only major cost failure item is the SMG pump. But, while they do fail, I don't think that it is all that common. Remember, online you hear more about problems than no problems. I know of about 5 longer term SMG M3s without any problems. None of the cars I know personally have had any problems.
'95 Alpine M3 LTW
'00 Dakar M Roadster
'02 Topaz M3
Red/White SRF #4 (Chassis 561)