I was perturbed enough by the 520/Surly discussion that after buying the Surly Disc Trucker to replace my 22 year-old heavily-miled but fatally frame broken Trek 520, I bought the new tan 520 so I could do a side by side comparison for a few months. It was an expensive comparison, I knew one of the bikes would go, but I needed to run them both for three months to see if 1) the Trek was really that much faster than the Surly, and 2) if there was a marked difference in the way four bags and gear feels.
I sold the 520 to a nice guy on ebay who was about to take off on an injured (Iraq--the sequel) vet tour. Maybe the Trek was a teensy bit lighter off the mark. It's lighter, and carries things higher, so it tends to favor two bags in the rear, but starts to show it age and workmanship when you put a couple of bags on the front. It may have been that the new Trek was just not like my 1991 Trek, but the decisions to go with the Surly is the result of the comparative speed differential (minimal), and the body workmanship, particularly on the stays. It's just a more solid frame than the Trek, and since I am without car in Seattle, the Disk Trucker is my new guy.