ctm - two-piece Unicomp caps in my opinion are comparable to some later production, original run Model M caps - the legends are less thick/bold and the top of the key caps are more shiny/slippery. They are still great to use.
The XT caps are one-piece, deep black legends, with a more textured key surface.
I wouldn't say all XT caps are better than AT - it depends on the date of manufacture of your keyboard and the amount of wear on the keys. 1984 and earlier tend to have the best quality in my experience.
Here are photos of the one piece keys on my NIB F77 (original), NIB PC AT, and little-used F122 (photos in that respective order):
For the F122 photo I have placed two Model M key caps (the "O" and "K" keys) on top of the original Model F keys for comparison.
The shininess is not actually in person but just due to the camera flash. It is difficult to photograph the keys 100% accurately.