Buy the new 520. Keep the Bridgestone. Strip the old 520 and save the parts for keeping the Bridgestone running as your back-up foul weather commuter. It's always nice to have a backup handy and you can tinker with it all you want. You can always sell the old 520 frame and any parts you don't want to keep and use those funds to buy the Brooks.

The above advice is of course worth exactly what you paid for it.