Ah... when you said Audi TTrs I misread and thought you meant the Audi TTs..
The TTrs seems likely to be a bit of a cracker with 340 horses available. I've not driven one of these. I've sat in a hard-top TT, but not driven one. I did drive a rag-top TTs (turbo 4-pot with about 290hp IIRC), a few times last year and my comments were based on that.
If you're not bothered about a folding roof, then the TTrs or Cayman may well be the ticket. Clearly with an SLK200, I was not looking for stellar performance. It goes well enough for what I/we need, but the main attraction was the folding roof and overall styling (inside and out). The SLK is not that commonplace either, so it still has a certain cache about it.
FWIW, we picked up our SLK in Sept 2008 and have really enjoyed driving it. It's a fun drive on local roads with the lid down. It proved to be a comfortable and highly competent GT on the trips to Le Mans this year and last. Even though it's not as quick as the larger engined SLKs, we don't really get left behind by the others on the sheep hunts, and if we're leading then folks haven't told me we were holding them up.
If you can pick up a lightly used 55 AMG, then I think you would enjoy it. They do not hang about and sound fab (I know I've been behind Mark's and his has the cross-pipe....
) If on the other hand you want a manual gearbox, then the 350 is the one to go for. Sometimes I think I would like a 350, but my better half is more confortable with the 200, which for us is pretty much win-win anyway.
Good luck in your search..