I don't care whether XT or AT (I'd need to convert to USB anyway), but I really want to have an F some day. There is still a (degrading) rubberdome Fujitsu at my workplace and I'm increasingly curious about how a Model F feels, especially since I have spent enough time to learn to love my Unicomp.
There are many opinions about Model F's (capacitive buckling spring) and M's (buckling spring over membrane). If you ask here, people will say they love them, hate them, love and/or hate one or the other, and everything in between.
But everyone agrees on this: They're significantly different.
To me, the F is faster, and its sound more metallic. The M takes a bit more effort, but makes the classic "thock" sound. I find both of them fun and satisfying, and I think they have more character than any other kinds of boards I've used. I switch to others for a while, for variety; but I always come back to the M and F—they're my "home base".
If you're serious about MKs, you really should have at least one Model F in some form. Both the AT and XT require you to adjust to older layouts. The closest Model F to the layout we know today is actually the 122-key terminal version:
Like the XT, it requires a special active converter to connect it to PS/2 or USB. But man, it's one solid board! And all the keys you need are there.
Unfortunately, people have discovered how wonderful they are—and an F-122 you could've bought for around $100 several years ago will now cost you at least $300. You may also have to wait for weeks till one is listed. But all you need is one of them—as my wife's dad would've said, "It'll see you out."