Well you are mixing "apples and oranges" if you will allow me a quick pun. Face Time is a built in Apple program that lets you chat with other Apple users via the mic and camera. It works with ANY Apple devices, so you can Face Time iPhone to iPad, or iPad to iMac, or MacBook to iPod Touch etc. All you need is for both users to have Apple devices with software that supports Face Time and most do.
ooVoo is a video chat program that is cross platform compatible, so it is possible for you to chat with Android users, Window users, or anyone who has a computer or device that support their program. It's a 3rd party program. Both users MUST have this program for it to work. Face Time is baked in to all Apple products. ooVoo has to be download, BUT many folks do like it. Skype is another popular option.
But again BOTH users must have the program on their devices.