Well, I'm an M3 owner, but not a BMW fan overall. I wouldn't buy any other BMW, to be honest. But a Merc even less. Having said that, the SLK is more comparable to the Z4, no?
You need to list your priorities (2 or 4 seats, etc), your exclusions (like force induction, automatic, etc), your price range, then drive the cars left to make a final decision. Good luck.