The S5 does look better than the RS5 and I reckon the reason for this is that on the RS5 the vents below the headlights are marginally bigger than on the S5 and in turn make the headlights look disproportionally smaller on the RS5.

Maybe this opinion will reverse when I see the new look RS5 in person but it's how I feel at the moment. Though unlike darkop I don't think anything Audi do could be a bad as what BMW has done with it's 1 series. That thing is just nasty.