The Sawed-Off is a powerful but shortened version of the Gnasher Shotgun. Instead of carrying that power for a longer distance, it widens the blast allowing you to catch several enemies in a single blast. The trade off is that you can't kill from nearly as far away but you don't have to be nearly as accurate with this weapon and if you are good with it, you can get several kills in a single shot.
This weapon is very useful in many situations and particularly for inexperienced players but it is generally less situationally appropriate than the Gnasher. For this reason, most of the time you are going to want to go with the Gnasher.
