For CUTs I think the 1/4 inch version is all you'd really need. In fact it makes even more sense to just use the two "quarter" plates and then weld a connecting strip. That will save 40-50 lbs of weight that will increase your lift capacity.
The 5/16 and 3/8 inch plates may make sense for big skidsteers but not for CUTs with less than 3000lb lift capacity.
If you are planning on welding some sort of implement to the front of the plate you'd have to reconsider but if you are just converting a bucket or grapple for QA use you don't need a heavy plate.
The two "quarter plates" (I cannot recall their official name) are also cheaper to buy and ship. You can get the top bar (I think I used either 1/4 or 3/8 2 inch stock which can be had locally at most any steel supply place).