Arx definitely felt similar but more refined. Arx felt slightly more linear, but with more depth and general refinement. Some cutscenes and whatnot. I didn't get too far in Arx, and only got 75% of the way through UU, but I feel like 80-90% of those who liked one should like the other in either direction.
Arx felt more challenging, too, in that it wasn't always so trivial to melee-kite everything with backpedal. Man, I need to play through Arx sometime, I got distracted by League of Legends is all.
Ah! That's more what I was looking for (not the that the others were unhelpful).
It sounds like, at least for a first playthrough, I would almost enjoy Ultima Underworld a tiny bit more, at least in the combat department. I greatly enjoyed the combat in Arx, don't get me wrong, but most of the time, when I play a game through for the first time, I like to be able to kind of breeze through it, just to get a feel for the game. I usually reserve the second playthrough for the more challenging stuff. At times, though, Arx was a tad more challenging than I wished on a playthrough.
But I'm rambling. Thank you for the reply. It was definitely helpful.