I think they are all about 1 CPM booking travel directly through them. The exception would be the citi prestige which is like 1.6 CPM on American Airlines only. Ultimately this is a convenience only option. To get the real value you want to transfer the points to one of the affiliated programs. UR has the best partners (United, BA, and Hyatt being the best, with SW being great if you have a companion pass).
MR has Delta and BA as well as SPG but the transfer is bad at 1:3 (they sometimes run a special making it 1:2 which is probably decent).
Citi has a bunch of asian and middle eastern carriers that are still great but you have to have the desire to fly international upper class or know how to use their programs to book through their alliance partners.
All will let you just book whatever travel you want at around 1 CPM though for convenience. There is huge value if you want to travel internationally in first class on asian and middle eastern carriers but realistically your better value is transferring to a carrier that makes it easy to redeem for domestic routes on partner airlines.