The SLK has always looked better than the Z4 IMO. You get a hell of a lot with the SLK, AMG or not. I have driven it, and it totally rocks!! The build quality, solidity and materials feel very much like Mercedes of yesteryear.
On a side note, I drove the new 2007 S550. BMW better really make the new 7 Series awesome, because the new S is amazing. Even the window controls are metal, and all materials feel perfect. The ride is tight, controlled and actually fun to drive. It is bank vault perfect!!