S5 if it's the 3.0t as it will keep up with the RS with a stage 1 remap. RS5 will probably need a carbon clean by now (if it's a pre-facelift) and will be down on power.
RS5 pre facelift looks better imo. Plus you get to have a real RS car. Yes, you can get similar power with S5 and a remap, but also the suspension is different, etc. and won't match the RS5.