Well, here's an honest opinion, since I had a 2004 350Z for 3 years, during which time my wife had a 2006 slk350, so I drove both all the time.
1. Power/Engine-wise, the SLK350 and 350Z are as peppy and quick as each other, however, I'm referring to the 350, not the 280,which is definitely slower. You can actually shift quicker in the 7speed SLK350 than a manual 350z (which I had)
2. Ride Quality: the same. (I'm talking about road harshness)
3. Handling: The 350Z with upgraded wide tires was miles ahead of the SLK. You could take a 90 degree turn and the Z would just hug without any bodyroll/drifting. The SLK would feel like its almost about to lose traction before the ESP kicked in. This is primarily because of the rollbar in the trunk of the Z.
4. Overall: The hardtop convertible definitely is a big plus on the SLK, as is the interiors, however the driving fun of the 350z with its gnarly sound (exhaust), is a lot of fun. Personally I drove the 350Z more, and I loved it. I had upgraded a lot of the interior, and I had 19 inch aftermarket rims as well.
5. Looks etc : I got more attention in the 350z than with the SLK, I think that has something to do with the fact that the Z was blue, and the slk a boring silver... I would have kept my Z for a long time, until it go so cheap (sold mine for 18k) that every kid on the block has one and is ricing it out...
