I've never driven an R8, but it sure is sexy. I would probably lean this direction since the benefits of the M3's more practical packaging don't seem to matter to you. A downside to both the R8 and Cayman is you can only carry one passenger. The M3 will fit four adults pretty comfortably even if you get the coupe.
If you plan to do a decent amount of track days then the Porsche would probably rise to the top of my list. It will be fun on a track and cheaper to maintain than the Audi.