Skype v Facetime, Mac to Mac

Just wondering if there has been a good test of this, or in anyone's experience here, which is a better(read: lighter) system.

Basically my girlfriend has poor Internet and am struggling to maintain a Skype conversation, and wondering whether it's worth her time buying the FaceTime for Mac App.

FaceTime seems much more barebones and possibly lighter, but I don't know if it is.