I think the safety and trigger guard on the 590 is metal (aluminum at least) as opposed to the plastic of the 500 (correct me folks if I'm wrong). A few other parts may be different. The easiest way to find out is to get a parts list for both guns. That'll tell you what is interchangeable. Always wanted to go to their armorer school, but it's even more elusive than Colt's.
Vigilantibus et non dormientibus jura subveniunt