If mods were available I'd say Skyrim in a landslide... Having had both on the Switch I will say that I put well north of 100 hours into Skyrim (I hadn't played it in about half a decade though) and got bored of Xenoblade 2 after about 30 (it felt like it took ages for the game to really get started and I just wanted to be let off the damn leash at some point), but from what I understand I'm more the exception than the norm for that game (though I imagine that's because most who bought it are really into JRPGs).
Really, one is very much a slightly more linear (but not nearly as linear as many of the genre) JRPG and the other is very much a western open world RPG where you can damn near ignore the main questline for ages (which I did). Whichever appeals to you more, go for it.
