If you carry a gun for self defense the question of what is a survival knife as opposed to a tool seems obvious. A knife you can use to kill someone is a survival knife. Hands down I would take a Fairbairn Sykes knife any day of the week. It has a thin long blade meant for stabbing. The handle is small and feels good in the hand. The history of these knives is well documented. They were used for special operations and undercover work in ww2 and later. There are many still available. The quality can vary enormously though fro ones make in Pakistan to excellent made in Japan. Price will be proportional to the quality of the steel.