There is a thread like this in the M3 Vs... section.
I've had both... I can say that S5 was better in the snow (obviously) and (to me only) it looked prettier. I loved the Led Lights... With Magnaflow exhaust and AWE downpipe, it sounded great... but it was slow and heavy.
If you're thinking about M3 vs S5... you should look into RS5 which is ready to be ordered and the base price is 68,900.
EDIT: Here is the thread: http://www.m3post.com/forums/showthread.php?t=664158