Although I was in my E39 M5 for over 3 years and the test driven X5M for only 30 minutes, I would not say that the M5 was better built. All of the X5M's on lot that I've seen have the basic leather which is vastly inferior to the M5's. The upgraded leather may be closer. The Nav, Phone integration and stereo are clearly superior on the new X5M. As far as fit and finish go, you have to live with a car a while to ascertain that. I like the Wheels better on the X5 M and they come equipped with better tires than the Continentals that were mostly standard on the M5.