If you're picky about hotels, MPLS is an annoying place to have to stay. The town is great and deserves an above average downtown property. The Hilton is about 5 years old and is passable. Rooms are very cookie cutter. Nothing special. Nothing wrong with them. Good location.
Pretty much the same story for the Hyatt. A little farther from the heart of downtown (few blocks).
The Whitney is unusual and has some nice rooms, but they're hit and miss. The Saint Paul has great character but is too far away if you need to be in downtown MPLS.
The Marquette has very large mostly corner rooms, is aging a bit, but some rooms I've stayed in have been great for the discounted price I get.
Hope that helps a little.

I agree pretty mcuh with the above post but I also like the Radisson Plaza Downtown and the Marriott City Center. WE have had wonderful rooms in both hotels and the location is fabulous if you want to be right downtown.They are both hooked to the skyway system and right by Dayton's Dept. Store as well as several other stores and restaurants. If I had a choice between the Hilton Towers and the Hyatt Regency I would probably pick the Hilton. Please feel free to e-mail me if you have any other questions. I am originally from the Mpls. Area and we still spend a lot of time there.