I have both an X5M and E92 M3, both are great cars and both have their advantages and disadvantages over the other. The X5M is super fast and has a lot of room for me, bring that I'm 6'6" tall. So, the wife drives the M3 and I drive the X5M, and I have an E39 M5 for weekend fun and track events.
If I had to pick one over the other, I would pick the X5M. Plus the X5M is faster!!!
///X5M...tracking a VIR....more so a parade lap with the BMWCCA Tarheel Chapter this past July.
2002 BMW E39 M5 : Evolve Alpha-N Stage 3 Tune, Evolve Headers and 200 Cell HF Cats, Bilstein PSS9 Coilover Suspension, Eihach Front and Rear Anti Sway Bars, SuperSprint Exhaust, X-Pipe, 19" BBS LM's, CAI, ACS Splitters, Icelink iPod, Valentine V1 Hardwired.
2010 BMW E92 M3 : Stock...for now.
2010 BMW E70 X5M: Stock.