I almost bought a 2013 S4 until I came across my M3 sedan and had to jump on it.
I like the S4, it's a great car, but it's definitely a completely different animal when compared to the M3. My one biggest gripe with the S4 is that even I would mistake an A4 for an S4 from a distance most of the time. The lack of distinction with the exterior between the models drove me nuts. It's just not aggressive enough when compared to the A4.
2011 M3 Sedan - Jerez Black, DCT, Comp, Navi, etc...