Some differences/advantages with the 520:

The 520 has a longer chainstay so that you have more heel clearance with your rear panniers and a more stable ride when loaded. It also has bar end friction shifters which you can repair on the road if needed. It's got an extra set of braze-ons on the underside of the downtube so that you can carry a third water bottle. It takes wider tires...up to 700x35 or 32 with fenders. You mention that your Quest has a triple up front, but what about the cassete. The 2007 520 has a wider range of gears on the cassette than the current Quest.