Untethered Angel is fine as a single, I think singles aren't normally great representations of a DT album and I don't think the strongest songs from the albums should be the lead singles in most cases. At Wit's End is my favourite song from the album and I don't think it would have been better of them to make that the single. Partly because, even though most people interested in Dream Theater are fine with longer songs, you do want to have something a bit more accessible as the single, and partly because you want to hold something back for the album.
Untethered Angel is fine (as said), but I distinctly remember first listening to it and liking it well enough, then when checking this forum I read a post that stood out to me which said something like "Standard DT single formula, cool second verse, quick prog instrumental in the middle that doesn't really add much", and I had to laugh because even though I wasn't that negative about it myself, part of my felt that description had hit the nail on the head.
However when I first heard Fall Into The Light, it was immediately more lively and exciting to me, the intro highlights the strong sound of the album, it is fairly accessible but still shows off more of Dream Theater's versatility. Untethered Angel was like "Ok, Dream Theater is coming back, here's a single that sounds like what you'd expect, now wait to hear what the album is like", Fall Into The Light felt a lot more like it showed me something slightly more unique and actually fulfilled part of the promise you hope for from a DT album, rather than just giving a feeling of having to wait for the album to hear the special stuff.
My impression is that Fall Into The Light was a fairly well recieved and successful second single. I think that if there was to only be one single from the album it should have been Fall Into The Light. If it was the first single, then Paralyzed (or Untethered Angel itself) could have followed it up. Paralyzed is a strong song in itself and could fulfill the role of a single well, but I think it might leave some DT fans with some questions because it is a bit too much of a "single" that doesn't highlight all of DT's qualities as well. That's why it's better as a second single imo. Fall into The Light is a strong song that's accessible in itself but also reassures fans and promises a great album, but still leaving plenty of the best stuff for the album.
Tl;dr: Fall Into The Light.