Based off of my own experience, each is their own thing. Unofficially Expanded takes the base CoC and either adds new scenes, or brings back cut scenes/characters, while editing some existing content. Xianxia and HGG seem (at least to my understanding) to use UE as a base for their content.
Xianxia adds soul force, a bunch of new characters and dungeons (not all characters are sexable), but has some problems when it comes to the coding and definitely some problems when it comes to grammar and sentence structure. It isn't horrible, but you may find that you have to reread scenes here and there to understand what's happening.
HGG has some good and some bad to it. On the good side, a lot of characters are interesting, sexable, and aren't children. There's also the slight addition of range to the combat system, where you can put distance between you and your enemy to make better use of your ranged weapons, but they can do the same. On the bad side, there's a bunch of child-like characters that are sexable. They try to skirt around it because they are either cursed, that's just their body type, or they are succubi cursed to look like children, but it's still just sorta there and not my thing. Thankfully (I think anyway, haven't touched HGG in a long while) there was a toggle to not have any sexual scenes involving kid-like things.
In short, UE is the most like base CoC, and Xianxia and HGG take UE in different directions. If you play Xianxia, be ready for numbers to start skyrocketing after a certain point. Did 2 ascensions (ng+'s basically), and when fighting Marae after beating Lethice, Marae had HP so high it simply displayed as 19422kk, and she regened somewhere in the range of 20 million to 200 million HP each turn. So ya know. Both your numbers and the enemy's numbers become insane after a certain point.
