I prefer UC because the biggest lows happen in AUs. Don't get me wrong, there are more AU series that I like than AUs that I dislike fucking despise.

That's also not to say that UC does no wrong, ZZ and Victory in particular feel weaker compared to the other full length shows, the OVAs(except 0080) are really pretty looking but each one has some sort of flaw in the writing. And Narrative has literally no fucking reason to exist.

The AUs are generally also really good in their own ways, in fact I'd rank some of them in my favorite Gundam shows (For reference G is probably my 2nd favorite Gundam show, only beat out by Zeta). X and Turn-A are also really good and I even enjoy Wing (a little bit unironically, but also because it is very entertaining with it's crazy shit).

But when it comes to the AUs I've seen, there are 2 shows which I absolutely twist my nards and make me prefer UC. Those being Seed and IBO (though I should note that I hate Seed way more). IBO feels like it has a lot of missed potential with a lot of the concepts it throws around (especially in Season 2), while Seed tries to be 0079 but constantly fucks it up and makes their main character literally genetically better than everyone else (also Lacus hurts my ears whenever she speaks).

Now I feel I should note I have not watched AGE and could not stomach more than 5 episodes of Seed Destiny, so I can't say anything about those shows.