Well the UTC is a purpose designed clipon, so it is optimized for operating in front of a day scope. UTM and Skeet are optimized as spotters.
UTC has collimating lens on the rear. UTM and Skeet have diopter adjustable lens on the rear. UTM and Skeet can work as dedicated scopes in a pinch, but that is not their primary design purpose. They can also operate as clipons in a desperate pinch, but again that is not their design purpose.
With the UTC, I can move it around from rifle to rifle and it is dead on POI to the day scope no adjustments are needed or even possible.
With diopter rear lens devices, you should count on needing to make adjustments every time you mount to a different rifle and you should expect to be off +/- 1 moa whenever you remount.
Now, as a helmet mounted thermal spotter, the skeet is probably the best we can buy. The patrol has a 12um oasys core, the skeet 640 has a 17um oasys core, but the skeet is half the size and half the weight. If you can find a used one for under $10k, that is a good deal. But for the new typical shown price, like $18k ... is the skeet worth 2.5x as much as a patrol? Not in my book ! Smaller, lighter and it has a laser.
The UTM is "in the middle" more useful in the secondary purpose of dedicated scope, still not optimal as a clipon, but a bit large for use on a helmet.
Used ones under $10k are interesting.
But for me, in the space the skeet and UTM occupy, I'd rather have a Patrol and I just ordered one to put my words in to practice!
In the past 4 years, I've owned an ATN ODIN (which I still have a like. For a 320 device it is still very used on my head beside a 14) ... and a Q-14 mod 0 ... which I sold, mostly because until the Universal Bridge came out I could not figure out how to mount a mum-14 form factor device beside a pvs-14. I know how now, but I sold the Q-14 mod 0.
Also have pas-29/coti and it is useful close in beyond 300yds everything is a blob, so ok for detection, but for id best within 100yds. But it gives you some fusion, see with the thermal, aim with the ir-laser, seeing the laser with the 14. I've been using it with good results for the past couple of months on my coop patrols. So I've run the gammut of helmet mountable thermals and completing the circuit now with the patrol, though 2+ years after many people went down that road
The UTC primary purpose is long distance in front of day scopes ... magnification 3x up to 20x. You get some pixelation above 15x but still no problem seeing and hitting targets.
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So UTC versus UTM and skeet is apples to oranges ... they have very different purposes. And throwing in the PATROL, I'd say the patrol does as well as the UTM or Skeet at everything except size, weight (skeet only) and built in laser (which is useful but not essential for my purposes).