It is basically all about the surface finish. We originally contracted with R. L. Adams for a foam board that had a smooth finish (better than fanfold, and approaching Depron), and we got a shipment last year that started well, but as we got about a third of the way into the pallets, we discovered that the surface finish had deteriorated to what we agreed was unacceptable as first quality MPF. We mulled over what to do with it, and finally decided to sell it at reduced price, as it's still good board structurally, and it's flat, just rough finish. On Grade A board, both sides have a surface finish like artist's sketchpad: not perfectly smooth like copy paper, but an even, very slight texture (typical styrofoam surface). On Grade B, one side has a "grooved" texture, somewhat like a wood grain effect, while the other has a "quilted" look. Tough to describe, and just as tough to photograph as you can see from the pictures below. (not easy to show texture on pure white surfaces). B grade foam is running at the high side of our thickness spec: (6mm +0/-.5mm) while A grade runs mid to low side. That makes B good for structure, as it's a bit stiffer, and the texture is immaterial inside a fuse, etc. Both grades sand the same, and in general bend the same (B being thicker isn't quite as easy to bend cold, but with a little heat rolls up just fine for fuses or wings). I need to qualify those statements, as we are talking about a fraction of a millimeter in thickness, so no significant differences, but I can detect a slight difference when building rolled and contoured planes like my Spitfire.
So if surface isn't an issue with your needs (making throwaway planes, combat, or full contoured and sanded surfaces), then Grade B is fine, and you save money. If you want the best surface, are doing profiles, for instance, or just don't want to do any extra finishing, Grade A is what you want.
At the request of several folk on the RCG Model Plane Foam thread, we now offer a mixed box: 8 sheets of each grade. That way you can selectively use each grade as you see fit.
For information of multiple box orders, click HERE