I don't know a lot about shuttle buses, but I LOVED the tour bus. We really got to know the guide/driver and she had awesome stories over the course of the eight hours we spent together. I wish I had gotten her name. It was really nice to have the same person for so long, and I think you get farther into the park than the shuttle buses will take you (though I'm not 100% sure of that).

My recollection is that you cannot switch between shuttle and tour buses.