Wow lots of opinions here mine is 19cf it is only for out of air emergencies to get you to the surface or to your buddys octo! plain & simple! if you get the 40 & I am not saying dont do it but you will find it too much to take with you & you wont bother most of the time! A 19 is small enough to have attached to your primary tank all the time on every dive you do regardless of depth its a nice back up that you will always use. Just my 2 cents for what ever its worth.