I saw this subject a few days ago, but forgot to read your post.
We've had good luck with the XT 12-34 cassettes. And we've destroyed five freehubs, so we generate some torque. But we're not super-beefy (200lb. male, 110 lb. female), so take that for what it's worth. However, I should add that one of our XT's developed a creaking while climbing in the second-largest cog. It hasn't gotten worse in over five years, but I keep an eye on it anyway.
I have, however, destroyed an XTR cassette on my single. Folded the three largest cogs. I was maneuvering the bike around a tight hairpin climb when all hell broke loose. Cassette failed and the rear wheel pulled a couple of nipples through my Mavic CrossMax rim. I was thrilled when Shimano replaced the cassette under warranty - it was the first Ti model, which had separate cogs and spiders. The replacement cogset is just like the XT, but with Ti cogs and has proven to be more durable.
I'd stick with the LX, since it's steel through and through. What does Santana have to say on the issue?