The Vltor billet upper is a heavier duty unit that has reinforcements built through out. It is also cut from a billet as opposed to a forging, so its dimensions should be exact, instead of just really close. There are people who feel that the billet is a superior upper for precision work due to its perfection with the billet. I'm not sure how much realistic gain can be expected in that arena.
For most people, its a matter of having an upper that looks different, or they can say is a billet.
Below is a picture of my Vltor upper on a SunDevil billet lower. POF is also making a billet lower. I've got some pictures of those I should try to post as well.