Both will equally WoW you! Both are very very popular, so you should definitely make reservations rather than just showing up at the parking lot.
Upper involves a couple mile bumpy ride along the riverbed in a jeep/truck. At the entrance, you walk straight into the canyon at ground level with the walls towering above you.
Lower involves a short walk to the entrance. Then you descend alot of stairs down into the canyon. Lower has a few narrower areas, a few other steps/rock stairs that you have to maneuver through. No big deal though.
I prefer lower just because it's a little more fun, as I described. But you won't go wrong with either. There are several videos on youtube you can check out.