Hatul
Contributor
Skip the Spare Air.
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You don't indicate the depth you plan to use it but assuming you don't want your pony to be the depth limiter, a 19 should work with not a lot to spare. It could still be packe and go with you for air travel without too much trouble. If air travel is not a consideration and you want more options for use, a 30 or 40 starts to make good sense. I can't think of any advantages of a 30 over a 40.
... and you thought you wouldn't get mixed opinions here?
Seriously (for a moment) ... the answer to most questions concerning SCUBA are going to be "it depends" ... with a bunch of other things to think about. That's just the nature of the activity.
First off, a .58 cfm consumption rate ain't bad for someone your size and experience level. And for typical recreational profiles, a 19 cf bottle will be all you need.
But whether that's the best choice for you depends on considerations of ...
1) how you want to mount it. For back-mount, anything bigger than a 19 cf bottle are going to impose some issues due to size and weight of the bottle. For slinging, a 30 or 40 cf cylinder are a better choice.
2) how you plan to use it. A slung bottle gives you some options you won't have with a back-mounted bottle ... such as the ability to pass it off to another diver in the event of an OOA. On the other hand, although a slung bottle virtually "disappears" if mounted properly, if you're diving in kelp frequently or tend to swim very close to the bottom, it has its own annoyances.
3) future goals ... as others have mentioned, if you plan to move into tech someday, you'll already have a handy deco bottle.
Personally, I prefer a slung 40 ... but it's partly because I'm used to carrying one anyway, and I already owned four of them. But I like the option to be able to manipulate the valve easily, readily see my pressure gauge, and hand it off in an emergency. Whether those are considerations that are important to you depends really on what kinds of dives you're doing ... and with whom.
... Bob (Grateful Diver)
If I wasn't confident in my skills I wouldn't be purchasing the pony.