I have a 2011 S4, spent months going back and forth and in the end I couldnt be happier with my decision, the 335 just looks dated imo, especially the interior, and the rear sports diff. option on the s4 makes a huge diff.
Antoher consideration, the 335 is getting a complete revamp, def. wouldnt buy one before that happens. now with the S5 unfortunatley it still has the v8 (i think) although I thought it was being ditched this year for the same engine as the s4.... if the s5 still has the v8 i would wait til next years mod comes out, they are doing a refresh that looks real sick and the s/c v6 is a thousand times better engine, including the dual clutch trans that comes with it.
oh one last thing, 335's are a dime a dozen, i see atleast 10 a day, booorring as fuck, i get stares and compliments on my car everywhere i go.