The wife has a MINI Cooper S with a few mods (17% pulley, CAI). It's really fun to drive, but my M3 is 8 billion times better (exact scientific measurement). Both will handle the twisties with ease, but the M3 has way more power. It's also a FWD vs RWD thing, so pick the one you prefer. I also find the M3 to be more comfortable.
If you had to compare the Cooper S to a 325, I'd say go Cooper S. I used to have a 325Ci and the MINI could take it with ease. Now, if you're talking regular Cooper, I wouldn't even bother.