While Glacier is my favorite National Park, Canyonlands is a close second, so in my mind you can't go wrong with either. Driving the Going to the Sun road in Glacier is a wonderful experience, although you may have to wait until July for it to open the full length.
As Lauren noted, the time of year might be a consideration. While Bryce might be cooler, the lower parks will be in the 90° + by early summer, which will make hiking tough. On the other hand, there is greater variety in southern Utah. Don't forget the state parks - Dead Horse Point, Goblin Valley & Kodachrome Basin all have much to offer.