@ekurburskiI think, I was in similar position a year ago, when I bough S670. My criteria in first place, was "versatile keyboard for home use, with many nice sounds, many accompaniments (styles) which can be customized... " -and I've got just what I wanted. The problem was, I didn't know how important certain "little things" will be for me. At the end, I've bought S775 several months later
edtek already pointed out all most important differences. I just wish to underline the importance of display size. Simply put, if you plan to customize voices and styles a lot (I know, hard to define "a lot"), then display on S670 is just too small and has too low resolution for comfortable work (i.e. when making many precise adjustments). I recommend, take a look on some videos and pay attention to display.
However, if you plan not to fiddle with "advanced" fine tuning much (who would know, huh?), then S670 is a perfect choice!
Finally, is S775 worth the price difference? Yes. Besides huge difference in display, bigger internal memory combined with faster USB port (compared to S670) is very usable. And not to forget: S775 has many so called "S.Art!" voices built in -it's the difference you can notice!
Decisions, decisions..