Well I found the ride to be better on 19" CSL's with PS2's than on the 18" originals with Conti's. I would guess that the Conti's are the main reason for that though, they just don't suit the Z4M at all. It felt like a different car with the 19" Michelin's on it, in terms of ride, handling and high speed stability.
With the same tyres on 18" or 19" wheels I think the difference in ride is going to be so marginal that it isn't worth worrying about. Unless there was a very noticeable difference I wouldn't swap the stunning looks of the CSL's on the Z4M. They should have had them fitted as standard.