It is a personal choice.
It is all relative;
No redundant air vs # spare air
If you have 1 min. of air it can save your life compared to nothing.
13 cu. vrs. 3 cu. This provides a wider safety margin, slow ascent, safety stop, deeper dives.
19 cu. vrs 13 cu. Even more of a safety margin.
30-40 cu. vrs 19 cu Even wider margin, deeper dives, longer safety stops etc.
I guess the best is a doubles set-up using the second 80 cu for a back up.
The advantages of spare air are, it is easy to bring with you so you are more likely to take it on every dive, it is light to carry, provides 10 to 40 breaths depending on sac. rate, depth, one breath of water and you're in uncontrolled panic, 10 breaths may be enough to get you to your dive buddy or to the surface, it makes you feel a bit more relaxed while you dive because you know you have a bit of added safety. Disadvantages, not much air, not much time, not enough for deeper dives.
Advantages of larger amounts of air. More air, more time, deeper safer dives, may save your life because of extra margin, etc. Disadvantages; bigger, heavier tanks, so you may not want to lug them around with you so you may be inclined not to bring them with you. Change trim and weighting, add to task loading, can cause imbalance while walking in and out of beach.
It is a personal choice. How many breaths do you want, will you bring it on every dive, are you willing to carry the extra weight and bulk, bigger tanks will change your trim there will be a little learning curb, how deep, etc.
I guess so but I doubt I'd be able to check that onto a boeing 747.
