I went on a TransAm trip last summer with a friend of mine. She was on a stock LHT, I was on a new 520. After seeing how both bikes handled the trip, I would happily trade my 520 for an LHT. Here's some pros and cons I found with the stock bikes:
LHT - Stock tires were terrible... soft, slow rolling, and prone to flats. The wheels were much sturdier (partly due to their 26" size), required less attention than my wheels did, and seemed to have a harder, better wearing braking surface. The hubs are a notch higher too.
The brakes were weaker than the 520, but the hood were much more comfortable. The actual on the road difference in braking power between the two isn't much.
It has a 1 1/8" threadless steerer and headset, which I think is a good thing.
Plenty of tire clearance with fenders. Rubbing will not be an issue.
Much, much better gearing and cranks. The cranks are square tapered, which I think is an advantage for a touring bike.
520 - Weak wheels. After the tour mine were pretty much shot. The seals on the front hub weren't that great (I think it was a fluke, not a testament to LX quality), and the rims were just bad. There are cracks around several eyelets and much more rim wear than the LHT wheels. I only weigh 140lbs, with a totally loaded bike I was around 240lbs total weight at the absolute heaviest. My rims shouldn't show that kind of wear.
I liked the v-style brakes, but wouldn't make it a deciding factor. They were very strong, but the hoods weren't too comfy. There are better v-brake road levers out there this year, which might make v-brakes a real option for touring.
The biggest shot against it is the 1" threadless headset. Really Trek, what were you thinking? The joke is they built a bunch of frames in '85 and just paint them a different color every year.
Pretty tight fender clearance in the back, at least compared to the LHT. Even some 32c tires might rub if things aren't set up right. I did manage to put 35c Invert Hardcase tires under Planet Bike fenders without any rubbing tho.
The gearing is notoriously high. Mine came with an Ultegra octalink triple, which I kept. I was well aware of the high gearing, but in all honesty, I left the gearing to see if i could make it cross country without pushing the bike. I did, but with a fully loaded bike weighing as much as 70% of my body weight, the Ozarks in Missouri hurt. A mountain bike spread on a square taper bb would be better.
In all both bikes made it across the country with only flat tires, eight flats on the stock LHT tires, two on the stock 520 tires, and one on replacement tires (yay Continentals and Schwalbes). The LHT wore a little better, mostly in the wheels. Her LHT still looks pretty new and my 520 is pretty haggered. With the XT components, stronger wheels, and lower price tag, I think it's a better buy.