For me, the RS4 seems too small....but that may be just cuz I love the fit of the RS6 so much....S5 sure is pretty....I was going through these same challenges and just decided to keep the RS6...what are you going to do with your RS6?

Bill @ Quiksilver
http://www.linkedin.com/in/billbussiere
Current:
2003 RS6 Daytona Gray/Silver/Carbon/V1 Hardwire/Lidatek System/Clear Bra/Med Tint/Plasma Fog Lights/Hella Smoked Tails/19" wheels/Supersprint exhaust