K770 is smaller, weights less and its battery lasts much more (10 hours vs. only 7!).

Maybe the camera is better in the K800 since it has a xenon flash which is much better than the LED one (K770)! Oh yes, the screen is larger in the K800i than in the K770i (2.0" vs. 1.9") this can be a plus using it as a camera too!