Facetime supports HD video, skype doesn't.
What mac do you have?
Facetime only good between stuff, kind of spotty in certain countries.
Skype: also be able to call phones with a decent international rate, not limited to stuff.
I use both, and both are great.
Facetime video quality is better than skype in my iPhone 4.
Who knows what Microsoft will do with Skypein the future?
I doesnt hurt to have and use both, there is always an alternative.
Sometimes I used skype to call a landline phone just to give the signal to my contact to fire up Facetime.
Facetime isn't free? Huh, I was under the impression that as long as you have 2 apple devices it just worked.