In-terms of performance, reading various comparisons from publications such as CAR, MT, etc the test data seems to be about even. I'm sure there is some variance depending on how each car is running and possibly the driver.
The M3 can be configured with limited options. Thus keeping the purchase price low. I can see this appealing to dedicated track users. However I suspect (and I don't have actual data to back my statement) is that most owners order their cars with most of the options. Therefore the MSRP would range from $72 - $75k USD. This is were the RS5 would be priced except for adding the ceramic front brakes at $6k.
The M3 being more pure could also be more rewarding to the driver especially for track day events. Where the RS5 with AWD and torque-vectoring would help the novice or casual driver. For some potential owners not having a MT would immediately exclude this type of car (as with a C63). Tough part is the lack of availability of any demo models. If one was to order an RS5 it would need to be on faith. I have test driven an M3 several times and know this is a good vehicle for me.
To an earlier statement about status of the M3 vs. RS5. I agree that in the US the M series has more of an appeal and following. Aside from the R8, Audi hasn't imported any of the upper line models. The last one was the RS4 and recently with the TTRS. It's more of question of which one would better fit the lifestyle and preference.
I am more aligned with BMW vs. Audi but I do respect what Audi has done over the last few years. They have some good products. Most likely I will stay with the M3 and submit my order in December but I have several months to research and decide. My post was just to solicit feedback from others who have looked at the RS5 or have owned one (overseas).