Keep the M6 and get the Ferrari! I'm saying this, with a 2006 M5 in my garage in addition to other cars.
I drove an F430 Spider F1 back-to-back with the Audi R8 V-10 Coupe. Two TOTALLY different supercars.
The Ferrari is all about emotion, passion, and begging you to drive it enthusiastically. The Audi is an EXCELLENT supercar, but doesn't beg you to thrash it around or treat it "rough" (i.e. enthusiastically). For me, it comes down to the entire supercar experience - the look, the sound, the impracticality, the lust of a Ferrari. I am choosing a Ferrari, and I won't look back.
FWIW, I think the R8 V-10 is probably the BEST execution of the modern supercar. But, it totaly lacks emotion and passion, which come through as the primary motivators behind getting the Ferrari.
It all depends on what you want dude. If you want an ideal supercar, it's hard to beat the R8 V-10. If you want a girl who knows how to dance, and will make you fall in love with her (including all her faults), get the F-car and don't look back!!!
PS: Expect some biased answers to your poll. If you posted this on some Audi R8 forum, weigh the advice with equal bias!!!