I bought the pre-filled version based on size and weight and wasn't disappointed. As many have said it's seriously overfiled with polystyrene beads but it's easy to remove some of the contents - I removed over half.
Of coures it has its compromises - polystyrene weighs nothing but doesn't hold shape well, so this is never going to be as stable as a beanbag filled with harder, denser "beads" like erm... beans, say ;-)
Of course if weight is no object buy the empty version and fill it with whatever works best for you - it's a well-made bag that isn't going to fall apart when filled with something heavy. Meanwhile I'l stay with the weightless polystyrene variety.