Umm, I own the PC edition and the characters are still visually a step or two down from what UC4 has shown, IMHO.
That's probably more about your tastes and such.
Unity's characters, except for the two leads, are near 1:1 implementations of scans from real living humans. They'r full of subtle touches of life in their faces and the extremely realistic lighting makes them look very real. Arno is very likely based on our original character sculpt (as he looks very similar) and I believe Elise is a heavily modified scan of a real person. My only other suspect from the supporting cast is Napoleon, who's probably also an original sculpt or a modified scan, as in his case Ubi obviously wanted to go for a likeness of the old paintings. Still,all of the characters are very well done, the facial animation is a big step up from AC4, and cutscenes are very movie-like, also because of the lighting.
UC4's characters however are probably all hand crafted, and as amazingly talented ND's artists are, you can still easily tell that they're not real people. The lighting is a bit simplistic as well, probably because they're still aiming for 60fps and have to take some shortcuts for efficiency. But once again, they're absolutely top quality, I don't know if there's any other game studio with such a level of artistry.
What they certainly have on ACU is that the game renders at 1080p with very nice antialiasing and there's a lot of high frequency skin pore detail in the textures, accentuated by the high specularity and the renderer brings out those details very sharply. I don't want to sound like I'm trying to degrade it, but it's a visual 'trick' to push the results. People are right noticing that it's reminiscent of FFTSW, where the same technique was used to 'sell' the characters. It's a very conscious artistic choice which you may or may not like, but it doesn't make the faces more real or lifelike or convincing IMHO. But that is good, the jump from previous games is still quite large and very near the limit of turning Drake unrecognizable and maybe even a bit creepy. Had they gone with scanned faces, it'd feel very weird and disconnected from UC3.
All in all I wouldn't say that there's a great gap between the two games' assets, the difference has to do a lot more with art direction.