M3 vs CSL = totally different driving experience, they're not even close. I've had both (well still have the CSL) and have used both extensively on road and track.

Differences at a glance

CSL is around 150kg lighter*, including much lower CoG (CF roof helps a lot)
CSL has around 17bhp more (thanks to huge induction kit, cams, ports, engine management)
CSL has totally different suspension
CSL has quicker steering rack
CSL has bigger front discs (most people fit AP 356mm + 6 pots though)
CSL has reprogrammed (more aggressive) software for the sequential shifter.
CSL has much better seats

* weight loss from stripped interior, extensive CF bodywork, SMC bodywork, thinner glass etc..etc (they could have gone much further though!)

Then there's the cosmetics.

Lovely jubbly.