i prefer the older supercharged version, but only for tuning reasons, but both are good runners, they hold their value pretty well. never owned one but took the old supercharged one out for a long test drive, and they are quiet a hoot to drive. good feel and feedback for a FWD.
its BMW servicing and standards so a fairly reliable motor. have a look on one of the clubs for common issues and faults,
personally i wouldn't go with the JCW but get a stock Cooper S as there are some really good tuning stuff out there. check out lohan for ideas i could have spent a small fortune at that place. if i had bought one.