Redundant Air
Perhaps the #1 hazard to divers, is that we aren't fish and can't breathe water. If we have a problem of any kind, the clock is ticking.
OOA is obviously the main issue. Yes, one can monitor air, but what if there's some kind of regulator-issue, such as a hose comes unscrewed or blows? A purely redundant air-source that is adequately sized to get you to the surface turns this PTSD-inducing scenario into a mere annoyance. Furthermore, redundant-air can save you from other hazards, by giving you extra time to deal with an emergency like and entanglement. Simply having it there, may also reduce panic. Redundant-air may come in useful in rescues as well.
Insurance: Yes, a pony bottle and regs costs $$$, but think of it like a form of life-insurance or PTSD insurance. It's also a (mostly) one-time expense. Think of it like a more affordable version of health-insurance. (some tips on saving $ below)
Sizes: Someone may give the run-down about how to calculate your redundant gas. I'm going to over-simplify to get people started, and I do have experience with most of these sizes. This advice is for recreational-only, not mandatory-deco-diving.
- 1.7 cu or 3 cu - This size may be a safety-hazard due to giving a false sense of security. It runs out of air at such an insanely fast rate, I really consider them somewhat dangerous. "Spare Air"s probably belong in the garbage.
- 6 cu - Minimum recommended size. Perhaps avoid depths above 75ft. Perhaps be ready to skip a safety stop. In my experience, I use about 100psi per breath at 8ft deep on a 6cu.
- 13 cu - This is a common travel size. It should handle most scenarios at recreational depths, but you should also be ready to surface without delay. You may need to skip safety stop if starting from deeper depths (120ft)
- 19 cu - This is my generally recommended size. I find a 19cu is no more annoying to carry, deal with, etc, than my 6cu. By the time you have a pony bottle, you might as well go 19cu. This should technically handle 130ft, with safety stop and increased air-consumption for many divers. (although maybe actually calculate this, if you're pushing limits)
- 30cu or 40cu - More air is always good. However "the 13cu on your person, is better than the 40cu you left at home." The problem I've noticed with larger sizes, is that divers get lazy. Most divers seem to leave their 30/40cu tanks at home or on the boat. Some divers claim they don't mind that size, but I'm just reporting what I observe.
- 80 cu - Ok, we're now in SideMount or Doubles territory, which is absolutely cool if you're willing to go down that path. There are manifolds, h-valves, y-valves, and more but that's beyond the scope of this comment.
Regulators: It's better to have cheap regulators for your pony, if you're on a budget, than no pony at all.
It just has to breathe, work, and get you to the surface. De-tune the regulators to prevent free-flow, there's usually a "screw" inside 2nd stage, where the hose attaches. It's okay to save money here, because the only purpose of this regulator is to get you to the surface. If your maintenance schedule runs behind a little ... again ... the important point is mostly that it works.
Practice: Take a few breaths from your pony every dive. Practice switching regulators on the shore if you haven't done it in a while (I was guilty of this), and remember to purge or exhale when switching. Switch regulators underwater too, at least once per dive. Also ensure the pony reg can be found, by feel (no looking!) almost instantly. Stuff the hose under some bungies, use a retainer, or whatever you need to do, to ensure the pony-reg is always in the same place.
Transfill: If you want to save money on fills and avoid VIP costs, just get a transfill-whip. It'll save you money in the long-run. (I'll probably post my $40 transfill whip advice in another comment)
Popularity: "I don't see divers with pony bottles!" This isn't a popularity contest. Maybe they can't afford one, or havent thought of buying one yet. Leave your ego at the door, safety is what matters.
NOT a Dive Extender: The pony should be there for emergencies, not to extend dives. Not to mention, it can be a hazard if you forget to top-off the pony. If you want extended dives, look into SideMount or Doubles.
Octo or No-Octo:
(Possibly Controversial Warning) In most SideMount, you have two first-stages, and 2 second-stages (Each 1st stage has one 2nd stage). No octo. Assuming your pony-bottle is adequately-sized, you may consider ditching your octo. More hoses, means more clutter/entanglement/mess, and possible confusion (i.e. grabbing wrong regulator). You could even re-purpose your "octo" as a 2nd-stage for your pony-bottle, meaning you only really need another 1st-stage and SPG (or button-SPG).