Trying to put the subjective issues aside, you'd probably choose the R8 for a few reasons. The V10 is a peachy NA motor, very crisp and linear up to 8k and the package as a whole is probably closer to the supercar experience you imagine than the 911, and has the added benefit of sounding just a bit special as well. The driving position also helps give the R8 more of the "this is a bit of an event" when settling in. The Audi trumps the Porsche inside, I don't think that's an argument even the most ardent Porker fan would try to counter. The V10 is different from the V8 in that it feels that it has a more pronounced *and* predictable RWD bias and would probably be quite happy being provoked if thats your thing. The R-tronic does what it promises, the manual with the snick snack gate suits the car more. Looks-wise, one of the issues is that not very much distinguishes the V8 from the V10 model (Spyder aside) so that dilution sort of spoils the image a bit. But then you could sort of say the same about the Porsche.
If you want to cross the UK in sunshine and rain at a phenomenal rate, you'd take the 911 every single time. It's freakishly good at moving quickly and the R8 is genuinely outclassed on the road if pace is a priority. Everything points to the Audi giving you the confidence to push on but the 911 actually delivers it, just massive grip everywhere, better brakes and brake feel and the PDK box is as fantastic as it should be, perhaps a bit too smooth. It's definitely a more "complete" missile than the R8...extremely competent, well mannered and quick. Like the Audi, it's a bucketful of performance you can live with day in, day out, the R8 being a gentler and more comfortable ride with the MR option in Normal.
Both are cars (not the cabs) that I revisited in the last 6 months and both are extremely impressive bits of kit, you don't really tend to walk away from cars like this massively disappointed. I have issues and reservations with both and my money would go elsewhere, but, *if I had to*, I'd probably plump for the R8...I'd be giving up a quicker and more polished performer in the 997 though for a more sporting, nimbler drive. It's hard not to make a little "o" mouth above 5k with the V10 in a tunnel of trees, but then again it's difficult not to grin catapulting from corner to corner in the Porsche and the mid-range punch would be missed.
Re-reading, I've pretty much ignored the cabriolet issue so bonus points to me but I'm in a rush now...