Have you driven an M3 before?...if you have, doubt you would be asking this question...
...now if your just looking for straightline speed or torque, then a Z06 would prob be the way to go...but for overall pkg, M3 tough to beat.
Yes a used one for sub 50K is alot of car for the money...this car isnt a torque monster but is well balanced and easy to drive...and with DCT lightening fast shifts.
1. check carfax(but not reliable)...get service history and make sure no paint work...get PPI from dealer...check DME for over-rev's and etc.
2. get low mile car with some warranty