When choosing your 2nd stage it is wise to ensure that it fits you! For instance, the mouth piece should sit comfortably and securely in your mouth and not rub against your gums. These stages are your primary and alternative sources of air so they should be reliable and not cause you any problems. Don’t buy the cheapest possible Octopus, remember this is for emergency use, and therefore it should perform well and reliably!
Larger 2nd stages will generally provide a smoother breathe where smaller ones are often lighter and take up less space.
Venturi control allows you to adjust the breathing experience, making it feel just the way you want.