Some more thoughts on 520 vs. BLT:
When you compare the bikes as they are sold the following differences leap out at you:
(1) Gearing. The 520's 105 setup isn't right for loaded touring, BLT comes with LX.
(2) Wheels. 520 owners have sometimes complained about wheel problems, BLT owners don't, or don't often. At any rate, BLT's a better bet out of the box here.
(3) Stem. One has to make sure the 520's not too low here. BLT's set up with threaded stem. But, the 520 can be set up for level riding if you buy the right frame size. BLT doesn't allow swapping on stems, either, so BLT doesn't clearly have an advantage here.
(4) Frames. BLT comes with a thick walled frame. According to BG's website, BLT's frame walls are thicker than Rivendell and Waterford. Trek won't release its specs so you can assume BLT's thicker too. Both Trek and BLT are made with 4130 ChroMo.
(5) Price. BLT is $500 more right away, and the price would increase to the extent you have the LBS build the thing.
However:
(1) LBS will swap out the 105's for LX at no charge on the 520 (here anyway). I mean both the cranks and the deraillers. One can get appropriate touring gearing, and the same set-up as on the BLT, for no charge.
(2) Maybe wheels would be a problem on the 520, maybe not. To be certain, one might upgrade the rims _at purchase_ and have LBS build up the wheel. Like $100 per wheel, but LBS might cut a deal if done at purchase. And one might rebuild only the rear, too. $200 max leaves one with at least $300 between the 520 and the BLT. Doing the rear alone leaves $400.
Then again, one might get by with the LBS just tensioning and truing the wheels themselves at purchase. They'll probably do that for free.
(3) Threadless is no big deal as long as the bike fits initially. Most of the complaints come from the Trek photo, which sets the bars too low for touring. But that's not a significant problem. Don't set the bike up that way.
(4) BLT's frame is probably thicker, and thus to that extent stiffer than the 520's. However, the 520's not just a theoretical entity. It's been ridden as a loaded bike by thousands of riders over several years. I've not come across complaints that the bike shimmies at speed, or is wobbly, because of an insufficient frame. In fact, the most common remark is the opposite: the 520's solid at speed even with a load. So, while the BLT seems to have a paper advantage here, the 520 frame's good enough.
I leave aside the rack issue since it's fair to consider both bikes as coming without racks for loaded touring.
In sum, comparing the two bikes is answering the following question: why pay more for the BLT? What, specifically, justifies this? That is not so obvious. In thinking about common comparisons, including some remarks on this thread, there's a bit too much throwaway reasoning here: one is built by a small shop dedicated to touring, the price of the 520 "approaches" the BLT, etc.
Well, when one does the math even with complete wheel rebuilds the 520 comes out about $300 to the good-- and again that assumes that one doesn't spend any money to have the BLT built at the shop. On the other hand, you'd have to pay tax at the LBS, however, and BG doesn't charge it for interstate sales, so there's between $100 and $150 added to the purchase of the 520.
So that leaves the price gap at about $150-$200, if you're building BLT yourself. If you're going to build the BLT yourself, the question, I guess, would be whether the BLT's frame is worth an extra $150 - $200. If LBS charges less for wheel work at purchase of a 520 than I assumed above the price gap increases. If you're going to pay for a shop to build the BLT you have to add that in too.
It seems to me reasonable to go either way here, with the purchase of the BLT being perhaps a bit on the high-ended side of things. But the Trek 520 with a gearing change and other modifications is by no means a clearly inferior bike, and it will likely be at least a couple hundred bucks cheaper, and perhaps more.