Much different animals. I have both a '08 M3 and '08 X5 4.8i (both loaded up). The X'r is by far the best SUV/SAV out there IMHO. However whenever I drive the X5 after the M3, it feels BIG and sloooooww. But if I leave the X5 at the airport and return a few days later, the X5 feels incredible so I think it all about perspective. I wouldn't trade either and wouldn't want to go without either one because they are both perfect for their intended purposes (e.g. when I have to haul all 4 kids, the X5 is always my choice over our Range Rover, and when I don't the M3 will always be my first pick (at least until it snows)).
I hope that helps.