If youre looking for something a bit rarer and iconic then go with the 355. If you want fast and reliable then get the R8. I just did a quick eBay search and turned up 50 R8's and only 19 355's. What about the Lamborghini Gallardo? A bit more sporty than the R8 imho.
The question not being asked is how you narrowed the choice down to those two cars - like, what was your motivation? Researching that might get you closer to the answer. To me, if the R8 is in the mix, why not something else, such as a Porsche GT3? That is what I would take versus the R8 for about the same money.
