I was in exactly the same situation as you were SteveS back in January. I decided on the V8, partly to do with it being newer/less miles, but also it is a generally easier car to live with day to day.
The RS4 is all about BHP, which is fine if you want to drive round at 4000rpm+ all day or on a track (where you'd get c10mpg), whereas the V8 sounds ****** awesome, can pull 6th gear from 15mph easily all the way through to 155mph if roads allow, and is more comfortable.
Yes the RS4 looks better, but in real world day to day living the S4 wins IMO.
Oh yes, I get c19mpg on most journeys, and up to 25mpg on a sedate cruise (c75mph). At 90mph NotNormal I'd get c20-21mpg.
I generally get c230 miles to the tank full, and I dont hang about either. Neither will you when you hear the engine...