The STS IS RWD too. If they're the same year, I'd choose the STS personally. They're a better built vehicle IMO. I didn't look at the STS because I can't afford one.
EDIT: After looking at pictures of both, I actually do like the looks of the CTS better, and how it's sportier. The STS is more luxury oriented, the CTS is more "sport" oriented. I may have to still choose the CTS over the STS just for that fact. The STS will be quite a bit slower too due to the 400+ pounds it's carrying around that the CTS doesn't have.