I'm not Brad, but I went to both Reims and Nancy this summer. I much preferred Nancy from the standpoint of aesthetics. Reims' architecture was much less interesting to me.
Reims does have the champagne houses (I don't drink), the surrender museum (less interesting than the other WW II museums I visited, but worthwhile if you're not doing a lot of others), and the Palais du Tau (worth seeing).
In addition to a generally more attractive townscape, Nancy has art nouveau and art deco buildings to seek out and an excellent art nouveau museum, the Musee de l'Ecole de Nancy. The tourist office has a little brochure listing buildings of interest, or in some cases short streets full of early 20th century homes. The Musee des Beaux-Arts is good, too.
Both cities have nice cathedrals. Reims wins on the stained-glass front, for me.
Neither is a bad choice, but I'd definitely prefer to return to Nancy.