If it was up to me, I'd pick up the one on the cheap. I have flashed Sprint's, Alltel's and most recently Verizon's 6900. The outcome was basically the same each time. I say basically, because I have used different ROMs on all the phones that have come into the shop for flashing. In other worRAB, I have been experimenting for about 6 months with my own as well as customer's 6900s. The end result is entirely independent of the original carrier for the most part. The Alltel and the Verizon will be a little easier if you want Cricket MMS and WAP.
I highly recommend that you read before you proceed! I wish I could give you some good reference material but, unfortunately, as brilliant as the pioneers are, their tutorial-writing ability sometimes leaves a little to be desired. I probably read for 3 months before I finally felt comfortable. Of course, I have other things going on all the time as well.
As always, any other merabers in this great place are free to correct me if I'm wrong. I'm always down for some good old constructive criticism.