Originally Posted by Lebowski

a rotor is a rotor basically, most are made of hardened steel, and brake pads are too soft to scratch them, they will all last forever xc riding unless you bash one on a rock.

larger rotors create more stopping force because of the added leverage i suppose. however rotors larger than 185 mm can have issues with standard quick releases, in other words they could rip them selves out of the drop outs.