Try as many as you can! Each will settle in the hand, pinch, pull, at different spots. They will also differ on the spread of the fingers, like how far you fingers are separated in the handle shape. This is what made a release comfortable to me. Also the thickness of the finger grooves. Will you shoot with gloves?? The size of the groove and spread profile will mess with that. Some are straight profile and others are curved. Some can do both. Try as many as you can. I have tried so many and wasted money buying and selling trying to find the right profile. Also pay attention to the neck length. Some are short some are long, it matters because it's going to change your draw length and where you anchor. I don't use a thumb anymore (shot it for 1.5 years or so), went back to an index, but I experimented with many when finding a thumb. Also, how it attaches to the D loop, can be important, especially if hunting. All the ones you have mentioned have the ability to hang it on the D loop either with a spring (UV/STAN) or how it loads the trigger (carter). Also do you want a lanyard? Some people are afraid of loosing them in the woods...a lanyard prevents that, but then you can't just hook it to the string and leave it there. Part of what I wanted was to clip it to the string and then still do other things with my hands (eat, bino/range stuff), but you risk losing them....(which sucks when drop $300 and lose it in the woods). Handheld releases are heavier than the index versions, so when they are hooked to a lanyard and then your wrist it can clang and dangle....not the best when climbing stands or messing with crap....
Nock on "Nock too It" is just a Nock on branded custom edition of the carter wise choice. It's a wise choice profile and trigger in a 2 finger version in custom colors. So if you don't want the 2 finger or don't care about getting the green color, you can buy the wise choice and it's the same release.
I have used Carters, Stan mostly. Shot a tru ball blade as well (it's pretty good, the flex version is more adjustable). I shot the UV at the shop.
My top choices:
Carter too simple: Was my main release for a long time. It has the shortest neck so it didn't change my draw length. Great trigger. Only comes in a 3 or 4 finger and has limited barrel adjustment. It's also only straight profile and only one size. Most carters are one size.
Carter wise choice: Like the too simple but longer neck. Has the circle finger whole for the first finger if you like that. It change my draw length by about 1/8-1/4 and so I sold it. Comes in 3 or 4 finger straight profile only.
UV Button: loved the profile, great trigger, pretty adjustable. IT'S by far the longest neck length handheld I have found. When compared to my too simple from Carter it lengthed my draw length by 3/4 of inch easily. This is why I chose not to use it.
Stan Onnex, both the clicker and regular: Can order in multiple sizes sm-xl, so you can really find the right fit. Same profile same release just has a click for the clicker version. The most adjustable I have found out of them all. Gets you a tru custom fit. Can be curved 2,3,4 ect. All the thumb button adjustability you want. About the same neck length as the carter wise choice. I shot the Perfex (Onnex predecessor) and the Onnex for awhile before changing to the Too Simple.
If I was to buy one starting out and had not been able to try the others I would buy the Stan Onnex clicker/regular. Mostly because it's the best value for customization and it has an amazing trigger. For the money it offers the most ability to fit it to your hand. If the others you can try and they fit (carters are cheaper) go with them. But if you have no idea and have to buy without seeing try the stan. The UV is great, but you will pay extray for all that other stuff to adjust it and you still can't adjust it to the degree of the Stan.
