Have driven both, love both. M3 is harder edged, S4 seems like a higher quality car. For the same money, S4 would be $5k cheaper and 1 year newer.

Anyone here owned both? How are maintenance costs compared to each other?

 

Both a PITA when it comes to maintaining. German luxury cars like to make you pay more than just the purchase price.

I love my S4. And here in Seattle, it's faster than an M3 from September to June.