Waste-O-Cash. The Speck is just a thick plastic overlay...and colored too. Why in the heck would anyone want to deface their MBP that way? The Invisible Shield is a thinner piece of plastic. It may protect it from a scratch or two, but you'd have to really abuse your MBP for that to happen. The only scratches my MBPs have is on the bottom where my screwdriver dragged across b/c I lost my grip. The Invisible Shield obviously does nothing for more catastrophic bumps and bangs.
My advise: spend your money on a nice padded sleeve or case that will truly protect your computer, and not just superficially.