No one really knows? I'd guess the Trek has shorter chainstays, shorter wheelbase, sightly quicker handling? It's OK to like the 520 more than the LHT. I like the Surly bikes for value, but honestly, they are far, far from the greatest bikes ever made. The truth is that many of the things that make a good loaded touring bike also make a crappy bike unloaded. The trick of the bike company is to cheat a little each way on the loaded/unloaded side and make a quality all around riding bike. Trek has been doing long before Surly. Trek made a great touring bike before some of the guys working at Surly were even born. But you can't go wrong here.... these are 2 nice, higher value bikes

As far as the steering tube and over bike fit.... that's between you and the LBS. A good shop can make get the right size frame and set it up for you. Relax, buy the Trek and never look back or feel remorse for it. Good luck!