One more vote for Arches.
But I will also say my parents just visited and they did both in one day. Now they are in their 70s/80s, so they are just driving and looking, so without any hikes or exploring, they were just driving and stopping at lookouts, but they got to see both and enjoyed them both in a day.
Canyonlands is actually divided into 3 sections, and the one most visit is "Islands in the Sky"... its a loop that's up on the top and has tons of scenic overllooks, and several options for short to medium hikes that will show you some great views, a couple of additional arches, etc. Needles and the Maze are back country and I think requires 4WD. Not feasible to combine with Arches or anything else. Canyonlands is massive.
Both are fantastic, though. I enjoyed them both more than the Grand Canyon.