I'm not sure a comparison between Skype and FaceTime is worthwhile. They're really quite different although both do video conferencing.

But that's where the comparison begins and ends. FaceTime completely knocks Skype out of the park on video and sync quality in my experience, but of course FaceTime would -- it's limited to Wi-Fi, it works only with recent Macs and iOS devices, so by self-selection its going to provide a good experience nearly all the time.

But I'm not ragging on Skype here. Yes, the video quality is ... um, a bit variable ... on Skype, but look at the range of devices it works with, PLUS it can do audio-only PLUS it can do text chatting PLUS it can (for a fee) call real phones PLUS (for another fee) they can give you a phone number ... it's a far more versatile program that kicks even iChat's rear end on everything but sync/video quality.

I have to use all three programs routinely: iChat for work, FaceTime for my increasing circle of iOS pals, and Skype for the PC laggards. I think you can tell which one I like best, but Skype is hands down the most versatile.