A cascaded purchase has three sheaves (8:1). A muscle box has 8 sheaves (8:1) and consequently much more friction. In fact, adding more sheaves in a muscle box is pointless, the extra purchase is lost on friction. The only thing better in a muscle box is that it is a neat solution. A cascaded purchase tends to become messy once it is unhooked from the halyard.
Because a cascaded purchase has less friction it is easier to operate. I am using a muscle box but I always have to pull the control line perpendicular with one hand and then quickly pull it with the other hand while releasing the “perpendicular” hand. This is to overcome friction. Just pulling the control line does not put the tension I want on the rig. A friend of mine, who uses a cascaded purchase, only needs one hand to pull maximum tension on his rig.
And as John already pointed out, a cascade is much cheaper. But if you insist on a muscle box: Harken does not make them anymore but RWO still makes them ( http://www.rwo-marine.com/ ). And if you are looking for quality similar to Harken try Sprenger ( http://www.sprenger.de/ ) (Sprenler is available through UK chandlers).