If you are planning to go from the Grand Canyon to Moab, then Petrified Forest would be a long way out of the way, and add way more time that you have available. As it is, just doing Grand Canyon to Moab via Monument Valley isnt going to leave you with much time for sightseeing.
If you have never been, then I would certainly opt for I70 through KC and StL. That route should save you some on tolls as well as you get farther east, although you do have to pay a couple bucks going through Kansas.
