I have never heard a good argument supporting a pony bottle. Not to be confused with a stage bottle (40cuft min.), a pony is virtually useless, it also creates drag and effort required for extra gear and associated clutter. In an air-sharing situation, one of you would be left with a very limited air supply (the pony) OR you will still need to have an octopus on your primary regulator 1st stage. Now you have three 2nd stages and two pressure gauges between the main tank and the pony. For comparison, an H-valve allows you one less 2nd stage and one less spg. Additionally, it allows you redundant access to your primary air (large) source should one of the regulators fail. The only failure that the H does not cover is; a tank fracture or catastrophic valve breach. Even a failed tank O-ring will not prevent a safe ascent it takes a surprising amount of time to vent a tank completely by opening the valve all the way on land (an o-ring failure will take longer to exhaust all your air and you would definitely be aware of the problem with plenty of time to surface from recreational depths). Regardless, a tank or valve breaking is astronomically unlikely. An H-valve allows you to connect two 1st stages to the tank, it also allows you to shut the valve to either post should one of the regs. have a failure thus preserving you main gas supply you will not vent all you air through the faulty regulator (either or both 1st and 2nd stage faults). With proper gas management skills (not breathing your tank down to 100psi), an H-valve will provide continuous access to a large air source where a pony will provide only a few breaths. You should practice operating your valves underwater, even adjust your tank position to enable reaching either valve if necessary. For recreational diving, if you can make a safe ascent with a pony bottle, you can make a safe ascent without one (whether you continue to breathe from a faulty regulator or you simply make an out of air ascent). Consider the limited air supply with a pony, a realistic working consumption rate at depth and you can see a pony merely extends your primary failed air source by a few minutes. An H-valve provides excellent redundancy with simplicity and minimal gear and thus minimal diver stress due to gear handling in an emergency. A good H setup will have a 1st stage on each post with one 2nd stage for each 1st - no octopuses. One SPG, one BC whip (dry suit whip if necessary, which should be connected to the post that does not have the BC whip for optimal buoyancy redundancy). Put your primary 2nd on the right post 1st stage, the BC whip and spg on the left with the other 2nd (for nicer hose routing). Finally, if you are diving with a ceiling (either deco or overhead environment), a true stage bottle is the best choice (That is, in the absence of doubles, independant or manifolded) but; an H-valve is the second best and certainly better than a pony which will provide only enough air to say your prayers or get you bent. An H is redundant for 99% of failures and 100% of likely failures. Remember, if even one little failure occurs, the dive is OVER even if you successfully handle the problem. Ponies are for posers. An octopus alone is nice for buddy breathing, but if your primary 2nd stage fails open, you will continue to lose air through it even if you switch to the octo. Your best bet is to breathe the freeflowing 2nd , relax and surface in a controlled fashion (at least you can use the escaping air). Again, it will take a long time to empty your tank unless you have 100psi in it. The octos primary function is to give to your out of air dive buddy.
