veganbikes

As someone who had a Disc Trucker and knows the 520 well I would go DT or LHT in a heartbeat. It is a quality bike with great component selection for touring. Decent steel frame and fork with a cost savvy but decent quality parts.
The Trek isn't a bad bike frame wise for sure but a touring bike with racks that don't hold much weight and an aluminum fork is a deal breaker. Also I personally would not want STI levers on my touring bike, Gevenalle levers are fine and bar ends are just dandy but I just don't care for STIs for touring.

I will say the 520 does have TRP Spyre brakes which are excellent and the external bottom bracket is slowly winning me over from square taper but overall they use a lot of cheaper parts. Surly uses Deore and XT derailleurs while Trek uses Sora and Alivio as a for instance. Also Surly uses Continental which are high quality and long lasting. I think my Disc Trucker just recently had its tires replaced after I rode on it for a few years and my friend put a couple tours under his belt as well. I would swap out the stock Trek/Bontrager tires right away which would add extra cost.

9 vs 10 speed. I am fine with 9 as I have 3 9 speed bikes (one of which I am in the process of building) but have been thinking about upgrading my touring bike at some point down the line to 10 or even 11 speeds but no need right now. The Surly is now 10 speed and that is fine, Trek is 9 speed and that is fine as well but obviously components wise I would go Surly because I would rather have an XT RD than Alivio.