This is not the place to say which game are better, but rather to share my experience as a player after trying both games.
I've start playing UATC since 2015 and known WoA since 2020. I would say the core gameplay are different between the two. UATC is more of a ATC simulation, while WoA is more of a airport managerial game, both excels in their own way. So the most logical thing to compare are their graphics and variety of aircraft and airports.
UATC starts getting really good after their detailed aircraft update. They managed to cramp PC level detail into mobile game, and the selection back then was unheard of. But they essentially stop updating from 2020 onwards. 2023 update was essentially reshade attempt and somehow it looks worse than the last one IMO. The only positive thing from 2023 is the livery mod for older UATC. Other than that, 4x speed is very handy, especially on this type of game. Camera angle is worse, now behind a paywall and I have not seen anyone using it. WoA graphics still not the best, but it always improving. Some of them in the recent updates are better than UATC right now, It also have much more selection of airports and aircraft as of right now(real airports too).
What I would like to see in next UATC update is new taxiway and air movement mechanism, essentially an ATC4 replica. On top off new aircraft and real airports. That would be very nice. Also variable zoom camera and remove binocular effect.