I don't know the differences between them but can pass along my observations. I toured with a friend last week, he using the LX and me using my XT crank. He had definite problems with creaking and squeaking which are admitted issues with his BB but he was also claiming he could feel some flex while climbing with his loaded bike. He is shopping for a new crank right now. Both my XT and his LX are touring cranks as far as chain ring sizes. They are NOT set up for MTB'ing. We carried comparable loads, obviously rode the same routes but I do weigh about 30 lbs more than he. I've never felt my XT crank flex or miss a beat.

I had an LX on another MTB and replaced it. I think my XT crank is heavier duty, I think it comes with a better BB but could be wrong and besides you can get BB's anywhere so thats easy to upgrade. I'm happy with my XT and would highly recommend it. It's by far one of my favorite cranksets no matter what type of bike touring or mountain. Not sure how helpful this is.

There are some very nice cranks for touring from Sugino, Velo Orange stocks their Cru models...all using square taper BB's which isn't a bad thing. You might want to check them out. Thats where my friend is headed I believe.