The other breech is plastic. The steel breech supports the barrel more. It also provides a tighter seal for the transfer port.All this is conducive to better accuracy and velocity.
Baker Air Guns sells a black anodized aluminum breech that is a close copy of the Crosman steel breech. It is lighter weight and has two set screws to retain the barrel instead of one. I have one on my 2260 and like it better than the steel one. It does not come with a bolt, so you need one with a screw in handle.