Ultra-G has a harder finish than the regular Ultra, but the packaging is different. Ultra comes in a nice retail package with a lanyard and a pocket clip. Ultra-G comes in a little baggie with no other packaging, no lanyard, no clip. Ultra-G also has "U. S. Government, NSN 234-234-2345" (or some other such ID number) engraved on it, which is cool in some ways I guess. I've heard that claim about the "selected" LED but I'm very skeptical that there's any difference in practice.
I like the regular Ultra a little better if the light is a present for someone else. The packaging is nicer and the clip and lanyard are useful. The Ultra-G's finish is more durable if you're going to bang the light around a lot, though.