I have used many different brands of cut off disc in a 4 1/2" grinder working as a Union Pipefitter.
The Walter Zip or Zip + is hands down the best cut off disc I have used.
They are directional,so mount them with the arrow pointing in the direction of ratation and they even last longer than is incorrectly installed, LOL.
$3.25 a disc but worth every penny.
If you do a lot of cutting, get a larger guard for your grinder and use the 6 inch discs, you will get a lot more feet of cut per dollar spent.