The Last Story is about 40-60 hours and in my opinion the best of the three. After all it's the ideas and variety in gameplay and story/characters that count (in an RPG), but Xenoblade is pretty amazing as well in almost any way (also gets my "best vg soundtrack 2011" award).
Xeno often goes way beyond The Last Story in regards of scale or "epicness" and helpful features and interesting extras, but the basic gameplay, battle system, and controlling of characters isn't as rich as what I found in The Last Story.
And don't listen to the other comparisons. Xeno lets you play as a group of teenagers (with 2 exceptions) and TLS has a more mature and mature acting party. If you're a teenie you might enjoy Xeno more. If you're older then you might enjoy the TLS characters a lot more.
And if you decide to get only one, you will miss out on one great RPG anyway.
PS. I'm in the last tower of Pandora's Tower. And it is indeed much like Zelda, but only the dungeon part (which is most of the game). I think it lacks an overworld that is more than just one building, and I have serious issues controlling my character when swinging on a vine or hanging lamp or hook or something. It's definately the most difficult of the three RPGs, because it's much more action and puzzle-based, and once your level is high enough to not get slaughtered by the monsters, it's still quite challenging to find solutions to the puzzly sections of the dungeons. Combine that fact with the time limit, and the feature "enemies can damage your items/equipment and make them useless until returning home to repair for money" and there you go.
I don't think it's on the same level as Xeno and TLS, but I like it still and will beat it soon, and because it's so different there's always the chance that you like it more than the other 2.

lol If you've seen MS's E3 2011 presentation with Ice demonstrating gears 3 than you would know what i'm talking about. Why so serious? lol