The very obvious difference will be strength and weight. The bigger tractor will have better stability due to size and weight, obviously more HP, bigger/stronger front end loaders with higher lift/reach, more 3pt lift capacity, more roomy operators area, etc. The added weight and usually larger tires will result in better traction, and better handling of heavy implements.
So, a UT will be basically better at everything .... Except ... If you have size & weight considerations. For me, I do commercial landscaping, so a larger, heavier tractor would lead to transport, maneuverability, and ground disruption issues. The sizes of the CUT'S vary widely, so if you have size limitations, there will be a CUT available to fit your size needs. For me, the largest of the CUT'S is what works best for me, though I'd like to add a smaller, SCUT sized TLB at some point for the real "access challenged" jobs, and also a bigger UT for some of my bigger jobs.
One other consideration is transmission options. Some people prefer old fashioned gear transmissions, or "souped-up" gear transmissions with clutchless direction changes (shuttle shift or upgraded power shuttles) ... And some prefer hydrostatic. Personal preference aside, each transmission will have its strong and weak points from a convenience and performance perspective, and depending on your intended uses, one transmission will be a better choice over another.. But the bottom line is, currently, hydrostatic is only available in CUT'S, and not in UT's. If you want hydro, you're obviously stuck with a CUT. If you want gear, then your options are expanded.
First figure out if you have specific size requirements (length, height, weight), and that will help you narrow down your options.