Devin Townsend wins this, but not without a fairly solid fight.
Steven Wilson used to hold the coveted position of "UMH's favorite artist". Porcupine Tree was pretty close to being my favorite band, but SW was definitely the best artist in my eyes, for his variety and general sense of songwriting. Before I discovered Devin Townsend, I didn't think I was going to find anyone better.
First off, let's talk about the variety between the two. Both of these guys are absolute monster musicians, there's no denying that. It really does boil down to personal preference, but I'm going to try to look at this from an objective standpoint. Steven Wilson is/was involved in 7 major projects. Bass Communion (Ambient), Blackfield (Alt rock), Incredible Expanding Mindfuck (more ambient), No-Man (pop rock), Porcupine Tree (Delicious metal-infused progressive art rock), Storm Corrosion (Nobody really knows), and his solo project (Less metal-infused progressive art rock).
Devin Townsend is/was involved with Steve Vai (80s guitar virtuoso metal), IR8 (Thrash metal), Strapping Young Lad (Industrial Rip-Off-Your-Head-And-Tear-Out-Every-Organ-In-Your-Stupid-Body metal), his solo material (Literally everything from 90s punk to ambient drone with some folk influence). Since Steven Wilson's projects all tend to sound fairly similar and within his basic style (Not that that's necessarily a bad thing), Devin wins this round based on his ability to completely warp the sound to a point where you could fool someone into thinking it wasn't even him.
Next category: singing. Steven Wilson is a master of making a beautiful, sonorous melody. Unfortunately, that's about it. Devin Townsend has a ferocious, throat-ripping scream, a huge operatic wail, and can still manage to have a more beautiful clean voice than our friend Steve. Another mark against Steven is his live performance. Sure, he sounds great on record because of all the layering, but live there are parts that he delegates to backup singers because he can't hit them consistently. Devin, in everything post-2007 that I've seen him in, absolutely dominates every note he hits. He was very shaky early in his career, but he's finally at a point where nothing can stop him. Everything from the ear-shattering screams of Deadhead to the emotional yelling of Numbered! is executed flawlessly, night after night.
Now for another aspect of their musical abilities, guitar. Based on creative riff-writing alone, Devin wins this without even trying. One only need to hear Deconstruction once to figure that out (Maybe two or three to cut through the layers and actually hear the guitar parts). Steven does have some very creative riffs, particularly on The Incident, but they just don't stack up against something like Far Beyond Metal. Steven also rarely shreds, but his occasional solos are slightly lacking in technical prowess. Devin doesn't explore the wanky aspect of metal very often, but when he does, it's obvious he knows exactly what he's doing and could run circles around most of the major guitar players of the modern age.
Something people like to laud Steve about is his ability to write a catchy pop song. Trains, Lightbulb Sun, Shesmovedon, all brilliant. If I had to give Steven one aspect of musicianship that just barely edged out Devin, it would be that. However, songs like Kawaii, Lady Helen, and Ih-Ah! just appeal more to my ear. Plus, I think Steven Wilson would shit his pants trying to come up with a song like Planet of the Apes.
Personality: A fairly unimportant aspect when considering musicianship, but Devin wins here as well. Steve can tend to be pretty pretentious (and some of his hardcore fans share that trait). Devin, on the other hand, is the most down-to-earth guy you will ever meet.
Anyway, yeah. Devin.