The difference is in the purpose of each blade. The flexible fillet knife is thin enough to slide easily between the skin and the meat of a fish or poultry. Boning knives have stiffer, thicker blades that can tear through the skin, making it much harder to separate from the meat. However, cooks who fillet meat, poultry, and fish, and who don’t want to keep switching from one blade to another, may prefer a hybrid boning-fillet knife.