NEED HELP- Catalytic Converter Front Gasket

They will both work. The OEM works with the OEM cat but you can also use the one that covers the whole flange (aftermarket). The aftermarket gasket will work better on non-OEM cats like the high flow cats because they use a very flat flange mating surface which doesn't have a recessed or rounded edge in the hole for the OEM gasket to partially sit into.

edit: Some aftermarket cats may have a flange that works with an OEM gasket but not all do. They may work if you tighten sufficiently.

Also, the after market gasket is easier to work with and install since it has holes in it that make installing the bolts so much easier while keeping the gasket in place.