The xt912 is older. There are two models of the xt912 which is the og RAZR. One has a bigger battery. Both are good phones. I have both the RAZR m and the RAZR Maxx. My wife has the regular RAZR. The RAZR m is a bit faster but has a smaller screen. Check out the RAZR HD maxx it is similar to the RAZR m software wise. The difference is the screen. The battery is a lot bigger and the internal storage. It all depends on what u want to spend. I believe if u go on Amazon u can pick up a RAZR maxx HD for $79 with a new contract, a little more if u already have a contract. Verizon only.
If I had to do it over again I would of paid full price for a maxx HD. But I didn't I upgraded to the RAZR M. I've seen RAZR hd ' s going for $400 and maxx hd's going for $500. If you don't want to spend much regular RAZR maxx's are pretty cheap $200 and under. Swapa.com is a great web site to buy used phones. ESN'S are verified before they are sold.