I booked the half-day rafting trip for a family group of 10, and looked at several different operators that all had good reviews. We picked Red River because they let everyone paddle, which isn't always the case, and their trip was estimated to take the least amount of time--meaning that you spend less time standing around waiting, since the actual rafting time is apparently the same no matter who you book with. I believe this is because Red River takes a smaller group to the launch point than some operators, so there's less of a crowd to deal with. There were just two buses when we went (the ride itself is a lot of fun), and the waits weren't long at all. The woman I spoke with on the phone (the owner?) was really helpful and patient--I liked her approach to things and that pretty well cinched it.
The paddle itself was gorgeous and great fun, and the highlight of our Zion-Moab trip. I'd certainly not have minded the full-day trip, but the half day was perfect. Also fun getting in the water. And our guide, Gabe, was fantastic--knowledgeable and good-humored (and you could tell the whole crew was having fun at their jobs). I'm afraid I can't remember the name of the young woman guiding the other raft, but she was great also. Highly recommended.
And I hope the crew eventually learned what the "blue bus" music reference was :)