XSE has a lot of upgraded features over the 1991. Not all are considered desirable by many. If the finish is a deal breaker then that's that. If it isn't and you want things like Novak sights, beavertail GS, three hole trigger, front cocking serrations and I believe an ambi safety (check the Colt website to verify) then it's a nice package for the money. I have a new Combat Elite which is, essentially, a two-tone XSE and it's a full feature gun.
That said, in the Commander, I'd probably get a 1991, upgrade the sights and enjoy the classic lines of that gun. Even though they really rubbed me the wrong way and I won't be doing business with them any further, PSA had 1991 Commanders for $599 last month - that's a great price!
ETA:
http://www.colt.com/Catalog/Pistols/XSE™Series.aspx