I don't know how you can answer this question when the V8 M3 does not come out for another year. Here however is a video of the 2007 RS4 vs. the new M3 CS:
Here is what the experts at Top Gear have to say about the new RS4. They also talk about the RS4 vs the V8 M3 at the end of video 2:
I took delivery of the second RS4 in the US and have taken out many M3's and 911's. However I don't know how quick off the line the new V8 M3 will be, but I cant wait to find out!