Definitely Z3....
I used to own a 1999 SLK 230 5-speed, and I don't know how to say 'lemon' in German, but man, this car was one. The trick top is nice, and I guess if you get the sport package (I didn't have it), the car looks cool, but there are several negatives - especially wit hth '99MY.
First of all, the Kompressor sounds like a tugboat - no joke. The car is also a little heavy for it's size, and it's an MB so it can't be as visceral a sports car experience as a BMW (I can't speak for the Z3, I've got a Z4). I had a few electrical problems with mine (lights, a/c unit) and all of the microswitches that operate the top had to be replaced. The switches are a known issue w/MB and may have even been recalled for '99. When one breaks, the top stops working - no matter what position its in.
I'm not going to say that I didn't enjoy the SLK after having traded a Camaro for it, but I will say that now I own a BMW roadster and can't imagine EVER buying another MB convertible. Yeah, the top is cool, but you don't drive the top - you drive the car!!
Good luck, whatever you decide!