I've got a few vintage Trek bikes in the stable: '83 720, '76 (employee-custom)520ish, '7X 710 (or something like that), '84 720, '85 620. Here's my take:
IMO the best touring bike ever made is the 720. But, and this may not be important to you, the seat-stay attachment on all of the vintage Treks was poorly thought out. There is very little metal there, so if you are big and carry heavy loads and like to tour off-road a bit (all of which describes me), you will eventually crack those pretty brazes. My 720 is currently waiting at a frame builder's shop for me to pick it up (new seat stays). If you are a smallish person who doesn't carry all the gear for groups of four when touring off-road, you may not have any issues. I did have one 720 that didn't fail for over 250,000 miles; I also had one fail after 10,000 miles, YMMV.

While the ride of the 720 is a bit more comfy and stable than the 520, it's not much of a difference. If I didn't have size Godzilla feet (16 U.S., 51 Europe), I would not hesitate to grab a 520 for loaded touring. In fact, when I was briefly without a 720 I did do a 750 mile tour on my 520. I did have to put the small bags on the rear, but I usually carry most of my weight in the front bags anyway, so that wasn't a problem.

I would get the 520 if I were you. In part because of the seat-stay attachment problem 720s (and other vintage Treks) are getting difficult to find. Also, there is quite a following for the 720 so you may be shocked at the price you must pay to get one. If you find that the 520 is just not the right bike for you there shouldn't be a problem selling it. Or, when you eventually crack those pretty seat stays, you can pay $200 and have both the seat stays and chain stays replaced to create your own 520-->720 by having the chain stays lengthened.