I had a go in a 55 AMG before buying my Z4M coupe.
I am a BMW man through and through which was kind of passed on to me from my dad. The cabins of the mercs just feel well... not quite up to scratch, the quality of mercs has gone seriously down hill to the point where 3 mercs were in the bottom 10 of the consumer survey. Although more recent mercs are better I felt like this one wasn't. Also I buy a car to drive it, so autos are a big no and all mercs have autos you will be very pressed to find a manual and they are no where near as good as the BMW manuals. It was also really annoying it is a cruiser, it wouldn't kick down, felt lazy and annoyed me. But the V8 was great! Fantastic sound track and I do actually like the look of them, the design is very nice much better than the previous model. But I suspect the 350 to be a lot more tame than the AMG.
Saying that the tin top was nice, it was nice to drive as a cruiser, seats were comfortable and it was well equipped. Problem was the Z4M c was better value for money and is rare and theres nothing on the road quite like it and when I had a go in the M that was it game over.
BMW Z4M Coupe with CSL wheels by
tom_scott88, on Flickr
BMW Z4M Coupe rear by
tom_scott88, on Flickr
Now everytime I open the garage and I am greeted by this bad ass I cant help but smile... then the noise... Its really hard to beat

Those pics are enough for me to never want to change it.
Although I would look at the newer 35i they are supposed to be very very good. Gorgeous interior and exerior just a shame it missed the V8 from the M3 if it had got that I would have been all over it. The only thing I would change it for is a 911 CS or C4S driven the CS quite a lot and cant get enough. BMW Ms are like the best of both worlds great pedigree with fairly sensible prices love them.