I have both devices -- and have used them both.

Similarities:

++ touch screen interface
++ nice display
++ good array of applications
++ both devices have publicly available SDKs

Differences:
++ 7710 has a smaller screen
++ OSes are different: 7710 runs Symbian OS. This is significant and makes the 770 more of a general purpose device. There is a larger set of core apps for the 770.
++ 7710 is a phone and has a camera. I never really appreciated the phone on the 7710 -- the face grease factor was too high.

At present, the 7710 has many of the apps the 770 has + a phone + a camera. However, in my view, the 770 has a more intuitive interface and is better for general purpose use.

-F