The reality is the RS6 will most likely be a quicker car then the M5. The RS6 will have an Audi claimed 570+ hp. Given that actually real world dyno's are showing the S8 actually has 575 hp's the RS6 will have 600+ hp.
The advantage will be AWD! M cars need to face reality that 500+ hp cars need AWD. Since AMG is offering the E63 in 4matic for 2014 I would guess M will need to do AWD to stay competitive. It's just a fact of traction.