I would lean towards the 520, the only reason being that I happen to like Trek products. I think either bike will be a good choice. One thing to think about is the Trek dealer network across the country might prove easier to access in the rare instance of a major product failure. I would think you'd have a harder time if you needed to find a Surly dealer. It's a small, possibly meaningless point, but I am just thinking of all the dumb stuff that would pop into my head if I were obsessing over the same choice.