To an average person a 550i really means its just another 5 series BMW. With all the negative hype the 5 series' been getting with the VW and Hyundai commercials but an x5? its one of the most respected luxury SUVs out there (yeah...SAV...) and buying a BMW is not just becuase we need a vehicle but we want a good one. you said you are planning on purchasing another car soon so I would think that the x5 would be a reasonable choice. I do see the 08 550 having more of the latest options but somehow here in North America, bigger and higher the ride the better it is for many, including myself.

P.S. BMW USA website under safety, the x5 acheived "best pick" for frontal crash but the 5 series did not. Especially if the vehicle is for your wife, its a consideration. Women perfer safety over the fun factor the majority of the time! (Motherly instinct? who knows..)

Cheers and good luck!