Two words: Ample space.
My friends tried my RS5's back seats and there is surprisingly a lot of room.
The fuselage/body of the RS5 is quote long (just the middle part for this equation). Because of this, the back seats have a lot of room.
Also, the sloping of the fuselage is not as pronounced/sloped as a Porsche 911 or a BMW M3 or even a C63 AMG. Nevertheless, even though it is not as sloped, the RS5 still manages to appease the eyes.
Also, because it is not as sloped, the driver and front passenger has a LOT of head room. This also applies to the back seats.
So, yes, there is ample room. And, without giving up the looks.
I had the chance to try the new 2013 S4 at the Audi Driving Experience.
The new S4 is basically a smaller version of the A6 (which I own) with less room, but with almost the same power.
And, you have to remember when you get the car, it is YOUR car and not your friends' car. Most of the time, I am presuming you will be driving it. For the driving experience, for the driver himself, the RS5 is better than the S4.
But, you get 1 fewer seat (the back middle).
But, you'll have to wait if you want to order a customized RS5, or about 4 to 5 months or even 6 months to be exact depending on your local dealer's allocation. Or, if you are not picky about options or if you are lucky to find a RS5 that is already in showrooms, then it is all for the better. I'm unsure about the wait for the S4. I am presuming that the S4 wait is the same as the S5 wait. If so, then it will be 2 to 4 months.
Between the S4 and RS5... Verdict: Go with the RS5.