I dive an AL19, which my air consumption calculations showed would be ample air for an ascent from my deepest planned depth while breathing at twice the rate of my usual high-end and still making my 2-3 minute deep stop and a safety stop. I also have an AL30, and if I were planning a dive toward the edge of my envelope, I'd carry that instead of the AL19. (In the meantime, the 30 is also small enough to unobtrusively leave behind the seat in the car for when I want tire air, hehe.)
Smaller than 19cf would not be sufficient to cover my expected depths, and I'm tall enough that smaller isn't necessary for convenience. Larger than 19cf is more than I need for any standard dive, and even the 30 adds enough that I wouldn't want to fly with it.
On the other hand, if I breathed a tank down as hard as mb's dad, an AL19 would not be sufficient air for a stressed ascent with stops from his maximum planned depths. He picked up an AL30, which should suit him. Of course, he was apparently thinking he'd use an old crappy unmaintained reg on it, which I have strongly advised against. If you're carrying a pony for backup, you don't need high performance, necessarily, but you do want reliability. (I used to like the AquaLung Titan, as it was a great just-works reg; unfortunately, they now downgraded it to use a hideous proprietary hose, which rules it out completely.)
Smaller than 19cf would not be sufficient to cover my expected depths, and I'm tall enough that smaller isn't necessary for convenience. Larger than 19cf is more than I need for any standard dive, and even the 30 adds enough that I wouldn't want to fly with it.
On the other hand, if I breathed a tank down as hard as mb's dad, an AL19 would not be sufficient air for a stressed ascent with stops from his maximum planned depths. He picked up an AL30, which should suit him. Of course, he was apparently thinking he'd use an old crappy unmaintained reg on it, which I have strongly advised against. If you're carrying a pony for backup, you don't need high performance, necessarily, but you do want reliability. (I used to like the AquaLung Titan, as it was a great just-works reg; unfortunately, they now downgraded it to use a hideous proprietary hose, which rules it out completely.)