The A,B,C ratings are for the quality of plastic. I don't have any of their puzzles, but I heard B, and C are generally not used for speedcubing. I'm not sure why the plastic is lower quality, or how it affects speedcubing, but to be safe, I would avoid it. If you are a collector, then I would say go for it. If I had extra cubing money lying around, I would probably just get one for the collection.
Also, as masterofthebass have stated, transparent cubes are not allowed at official cube competitions.