Welcome, Mike!
Gosh, that's an interesting question. I'm not sure what the "A" in "XA" stands for, but I'm somewhat confident that the "T" in "XT" stands for "Tyros," and that the number may stand for which Tyros model the file is designed for-- the original Tyros (now often referred to as "Tyros1"), or the Tyros2, Tyros3, Tyros4, or Tyros5. I think the primary difference between the various XT versions is in the voices which are specified within the song or style-- especially the Mega Voices and other "featured" voices which are available on each model-- and to a lesser extent perhaps some of the effects as well, although I think there tends to be fewer changes in those from model to model.
Anyway, you can probably buy any of the versions if you're capable of, and willing to, spend some time tweaking the file in a utility program to "revoice" it as needed so it uses only voices which are available on your keyboard model. I think the Mega Voices are usually the hardest ones to revoice, because they're programmed to select from different samples based on the MIDI Note velocities, so simply replacing the specified Bank and Program with values that work on your keyboard might not produce the results you're hoping for due to the way the Note velocities of the original Mega Voice had been carefully chosen to trigger specific samples of that voice.
What this means is that-- if you want to get a version that includes chord data and lyrics data, then revoice it as needed-- you'll probably have less work to do if you get the lowest-numbered version which includes chords and lyrics, because that version will most likely make use of the least number of Mega Voices.
In any case, I would recommend buying just a single song to start with, so you can see how well it works as is, and how much work is involved in tweaking it as needed. Then you should have a better idea of whether you want to buy more songs in that format or just stick with the XA version.
By the way, you can use the free MixMaster program (as well as others) to add Yamaha chord data and lyrics data to any MIDI song file, although adding that data to a song file that doesn't have it might be more work than simply revoicing a file that already has chords and lyrics included.
Also, you might want to look into Yamaha's Chord Tracker app, if you haven't already done so.
