All answer posted make sense, after is a personnal preference, you need to experience different configuration and see what you like. You should ask yourself one or two questions being :
Why do you need specificaly a 40 CF ?
What's the deepest you will go ?
Calculate sac and RMV and choose cyclinder after.
You instructor will help you out.
Be safe
Why do you need specificaly a 40 CF ?
What's the deepest you will go ?
Calculate sac and RMV and choose cyclinder after.
You instructor will help you out.
Be safe