The major difference is the time and amount of water absorption. Pellets will absorb twice as much water as shreds, but also take much longer (as mentioned in the other thread). That makes sense, once you take into consideration surface area v. volume - just like crushed ice melts faster because there's not as much to get through, the shreds absorb water faster because more area is in contact with the water.
I've never heard of nor seen crumbles, but if they're even smaller pieces than the shreds, then I image they would absorb water even faster, but not nearly as much.
Another difference is in price. Pellets, from what I've heard, are the cheapest.
The final difference is in safety, but I want to address that specifically to one of the posts above me.