I've been looking at youtube clips with M3 vs RS5. Seen 3 of them so far. here are the results.
1) was rally about the Cadillac, matching it against the RS5 and M3. Ultimately, the M3 remained a benchmark, then came the Audi, then teh Caddy came in third (transmissions is way behind the other brands). But he said it's a strong competitor otherwise.
2) another matched the RS5 and M3 evently, saying that it would come down to the skills of the driver
3) same for the third ... and they were driving HARD! They pointed out the strengths and drawbacks of each.
Cost wise, the Audi is more expensive. Taking nothing away from BMW, I've always said that the M3 is a "poor mans sportscar" (that title belongs to Chevy and Ford now). The M3 has history, and let's be fair. There are a LOT of good performing cars, but they change badges, rename badges, discontinue badges, etc. The M3 has stayed "M3". That has helped its iconic status. Don't believe me? Ask your fathers.
It comes down to personal choice though.