The advantage to Skype and Tango right now might not be as amazing call quality - but you can do it anywhere and it's device agnostic.
The DISadvantage to Skype and Tango is that both parties need to be signed in, like an IM client. For most of my friends and family, I'd have to call them first and tell them to sign in so we could video chat. FaceTime, on the other hand, works like a phone call and goes directly to the device, no sign in required. THIS is what makes FaceTime stand out.
Perhaps AT&T is trying to make FaceTime over cellular an LTE only feature for the iPhone 5? No additional charge, but you have to be on the LTE network to use it?