the advantage of getting the 750 over the 910 are great unless you need the vocal harmonizer that's in the 910 and not in the 750.

The 750 allows you lo load up to 64MB of custom Yamaha Packages. Currently these packages are:
Celtic and British
Brazilian
Eastern European
Spain
France
Many middle eastern packages
and there will be many more in the future

The 750 allows you to play .wav files using multipads
The 750 has a better internal sound system.

The 910 is better if you play midi files and you need to see the lyrics on a big screen.

The 750 has the new modeled distortion algorythm which is not in the 910 which makes guitars and electric pianos sound more realistic.