I have a '13 on order, so I guess I'll chime in here given that I had to make a decision between ordering an RS5 and an M3. Previously I had a '10 S4 and liked it quite a bit, so I was definitely biased toward an RS5. Here's why I chose to order an M3 instead:
- No test drives - RS5s have only now appeared at dealers and allocations are going fast. Ordering months ago to receive one now meant you were guessing at the driving dynamics. You didn't have to take possession if you didn't like it, but that's a long time to wait. I guess you could order both and cancel one, but that just seemed like being dishonest.
- Priced at MSRP - Most of us here didn't pay MSRP for an M3, but good luck finding a discount on the RS5. This means you're really comparing invoice + $X for an M3 to MSRP for an RS5. The MSRPs are fairly close, and the invoices are fairly close, but MSRP on an RS5 is substantially more than invoice on an M3.
- Carbon build-up - The engine is direct injected and previous direct injection naturally-aspirated engine offerings from Audi have unfortunately suffered from carbon build-up that robs the engine of horsepower. The S5 V8 and previous B7 RS4 were known for this.
- Weight - The RS5 is roughly the weight of the S4 and I can tell you from driving both that the M3 feels lighter. This is partly because it actually is lighter, but likely also because of better weight balance. The RS5 might hide the weight better than the S4 (and I hope it would), but it's still going to weigh more.
So in the end you have to pay thousands more for what is essentially a comparable car with a few negative points. I will say the interior looks a lot better, and even the S4's interior is (IMHO) more aesthetically appealing than the M3's. If money is no object and you want a more exclusive car, I'd say go with the RS5. Otherwise, I think the M3 will still end up being just as good or even better.