why?

high iso performance is off the charts, and its FF ftw.
if you dont need it for track stuff, then get a 5d. I have a 40d, and a 1dmk3. if the 40d had high iso performance it would be an awesome camera. but it doesnt, sadly. so i rearely use it unless my other camera is dead.
50d does have the new LCD screen. so thats cool