I didn't vote because I don't have a good basis for comparison. I have a good amount of experience with the 520 and none with the LHT. However, from all indications folks that own LHT seem to be pleased with their choices.

I have two Trek 520s, a 2004 model and a 2007 model. Both have seen a moderate amount of rough use:
- The 2004 model went on a ~2600km trip from the Hungary/Ukraine border to the Volga River. It also went on the Pamir Highway and into China (ridden by a friend). I just finished riding it last week for ~300 miles across Arkansas.
- The 2007 model went on a ~13000km trip from Amsterdam to Vladivostok, ~4000km across China and numerous other places. This July I rode it ~1350 miles on the Great Plains from Abilene to Minot. It has also seen a fair amount of daily/commuting and shorter tour uses over the past decade.
I am large and heavy cyclist and upgraded: wheels, saddles and pedals. I added front racks and also broke/replaced the rear rack on the 2007 model. I've worn through multiple drive trains replacing bottom brackets, chain rings, etc. I have not had occasion to care whether my bike had 700c or 26" wheels. (I rode a different 26" mountain bike across South America, but even there the issue wasn't availability of basic parts - but access to higher quality tires - in which case I was bringing extra tires anyways).

From my personal experience, I don't you'll go wrong with the Trek 520. I just don't have experience with the LHT to make an equivalent comparison.

I have wondered about sometime replacing my Trek 520s for a new model. I don't have a particularly good reason to do so, though I have become more of a fan of disc brakes (because otherwise I eventually wear through and brake rims). The other change I might eventually want to make is replacing my hub/rim combo from 48-spoke Phil Woods rear hub to a 40-spoke DT Swiss Tandem hub. The reason is more based on experiences with the hub failing in the field and not being as easy for me to service (discussed on BF: Touring hub reliability: Phil Woods and others?).