I ve never driven a s4/s5 but I think audi has its unique good looks. is it better looking than BMW 3's? Not in my opinion... different looks. BMW is more edgy and angular, audi is more flowing. it is personal preference, since both scream "luxury".
as for the interior, I just read too many car mags that praised audi's interior quality so I'll give audi the edge.
but we dont buy 50k+ cars for interior alone... car's performance is what we pay big bucks for. in handling and performance I'll give a significant edge to BMW. most articles that I read states how superior BMW is when it comes to handling. almost all the ariticles by different mags has similar comments about audi's handling as 'detached' 'dull' or 'sluggish'.
I do want to say the current S4 is a ass whooper. it recently beat 335i in a car and drivers comparo, albeit being almost 10k more expensive. well, it beat 335i by a very small margin. s4 is a only car that can say it bested a 335i in a comparo...
so all in all, what is my point? dollar for dollar, BMW wins supreme over audi still.