I pretty much agree with Anonymoose. Rare/Ex doesn't really explain it as well as Unique/Untradeable do, but somehow Rare/Ex just sounds better. Maybe it's 'cause they're shorter? I'm not sure.

Personally, if I had to choose between the two, I would keep them the way they are. I prefer the U/U because it's much clearer as to what it's referring to.

FFXI + FFXIV are the only MMOs I've ever played; perhaps someone who has played other MMOs could chime in on what other MMOs use?