FaceTime only works between iDevices and even the, both ends have to be on wifi. Once ios6 comes out, you'll be able to use FaceTime over 3G or 4g....but only with a new iPad 3 or iPhone 4s.
Skype will do it all....wifi, 3G, 4G, iPhones, iPads, androids, pcs....anybody using Skype.
I personally don't get the appeal of FaceTime...at all.