human33, If you can keep only one, choose the one that fits best. If they are equally well fitting, choose the Cannondale. I admit being biased towards Cannondale, just so you know. That written, I like the 520 and consider it a premium loaded tourer.
Touring bikes regardless of their frame material are generally stiff feeling when riding unloaded, they have to be in order to control the weight of added touring gear. Decreasing tire pressure increases riding comfort when unloaded. After almost twenty years riding Cannodale road bikes I've never considered them fragile, in particular the '80s models.
Both bikes are keepers and there's a lot advantages to have both.
Brad