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.
Good luck.
EDIT: Here is the thread:
http://www.m3post.com/forums/showthread.php?t=664158
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Current: 2014 991 Turbo RS
Past: 15 GT3, 17 R8 SC, Two 16 M4 GTS’s, Two 18 GT350R's, Two 13 RS5’s, 13 M3, 10 R8, 15 M4, 09 M3, 04 M3, 09 S5, 02 M3...
It all started with an RX-7 (FD).