If you want to win you are going to need a good helmet and vest, so when looting untouched equipment always grab the highest level possible. But if after a fight you find a damaged level three on the person you took down it can be difficult to know what to pick up and what to leave. The general rule here is actually pretty simple. Always take a level three helmet, even if it is close to being destroyed. A level three will always save you from a headshot with the M24, and is the only one that will do so. Therefore it is nearly always worth it.