Right now the 750 looks to be GSM only. That could change at some point, but no one knows exactly when that will be. As far as using as a modem, your 650 will already to that with
PdaNet
I can't offer advice as to whether or not you should upgrade. You will get higher data speeds, and more memory on the 700 series, and then the Sprint Vs Verizon issue comes down to how good each is in your location. . . . Sprint seems to be a little cheaper, but Verizon seems to be a little better at Customer Service . . .
Hope this helps . . .