We've done both and I'd recommend taking a towed. Rental cars are a PITA. Yes, it's nice to drive someone else's car and not having to pull something behind you, but you have to spend a lot of time getting the car and then returning it when you're planning on leaving. We rented ONCE and said never again. You'll not enjoy the Yellowstone trip in the moho although many people do. You won't be able to stop at all the pull outs as many are accessible only with a passenger vehicle. The Grand Canyon stop will be easy. You don't need a car there. The bus will pick you up at the campground and you'll be able to change busses as necessary to get around. I have some pictures in my blog. Enjoy the trip...Dennis
