I liked them both a lot but I'm kind of leaning more towards S/M.
I played these games all the way through about 4-5 times. I got super into them and wanted to play them over and over again.
I did not feel the same way about US/UM. Maybe it's just because I played S/M so much, and not much storywise changed but I don't think that's the case. Honestly, while not much changed, I also felt like the things that did weren't all that important. I felt like they didn't actually listen to what people actually wanted, or their criticisms with S/M. Some examples I can think of off of the top of my head, more/varied clothing (long pants), national dex, festival plaza (sorry, but if I just want to wonder trade why do I need to load this up, then connect to the internet everytime I open up the plaza), and this might be more of a personal thing but why create the character Moan if you aren't actually going to use him? He's the actual founder of team Aether and disappeared through a wormhole. It feels so useless to add this character that literally impacts the story, but not has him be an essential character.
So, while I feel like it would have been nice to get some variations with the story throughout and not just at the end, I feel like it's the creators not listening to what the consumers wanted for the new game that really turned me off from the US/UM.
I'll try and play it again at some point to see if I overlooked anything, but from what remember other than some totem pokemon changing, the game becoming more difficult, and some stuff near the end of the story, I feel like it was just a retelling of S/M.