Looking at getting the M and looks like there are two models and the only difference is battery size and color? Looking for clarification!
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.