I made my reservation through
http://www.lakepowell.com
because the website states that Lake Powell Resorts & Marinas is managed by ARAMARK, an authorized concessioner of the National Park Service, Glen Canyon National Recreation Area. I picked up the ticket inside the Wahweap Marina Resort reservation desk.
I did not research the other tour companies. An internet search might reveal some reviews of the other tours. They all seem to be similar and depart from Wahweap Marina.
I posted some photos of my trip to Rainbow Bridge here:
http://www.pbase.com/aw11mr2/rainbow_bridge&page=all