Katleman mentioned hiking -- we found that the hiking in Grand Teton was more mountainous (i.e., fun) than the hiking in Yellowstone, which is generally not as steep. So it depends on what you like. We spent a week in Yellowstone and four days in the Tetons, which seemed about right for us. But if you are not a hiker, or if you don't like steep terrain, I would tilt toward Yellowstone. (I don't want to give the impression that we are mountain climbers -- we are not. We are mountain walkers. Big difference. If I have to use my hands to make forward progress, that is too tough for me.)