I agree with Daniel Beaver's assessment. XT's are built ever so slightly better, but it's actually not really worth worrying about, unless maybe you're planning on frequently dropping your keyboard off the roof. The metal plate in the AT is really all you need in my opinion, and the metal bottom on the XT is simply overkill. I like the feel of the XT switches ever so slightly over the AT's, but again, very similar. I do not like the cursor pad placed right next to the typing area on the XT. I like that small gap there that the AT has. I actually owned one of the first XT's on the market (wasn't even an 'XT', but just a 'PC', without any hard disc drive). I fell in love the that original keyboard, but very quickly adapted to the AT layout when that came out. After using the AT and Model M layouts for so long, I think it would be hard for me to go back to the XT layout, even with some modifications.
One weak point on either the XT or AT are the grips on the left/right side for opening/closing the legs. They tend to break off easily, if the keyboard is dropped and hits on it's side. Even without the tabs, legs can still be opened and closed, with just a bit more effort. If there was one thing about the keyboards that you could call a design flaw, that would be my nomination.
