Many good answers here and many stupid ones also. For no deco diving in 100 feet of open water a 13 cu-ft tank will get you up. Even if some people say that is not enough air to do much of a safety stop, so what. This is an emergency! If the crap hits the fan and I have to come up on a pony, then i'm really not that worried about blowing off an OPTIONAL safety stop. Compared to the alternative of having no back-up other than a buddy, this is a far better alternative.
I find that people that initially buy 30 and 40 cu-ft ponies, choose to leave them on the boat if the dive is just 70 or 90 feet because they are heavy and have drag. A 13 is very light and has not much drag. The 19 is just a little heavier and is longer, but I suspect the drag is very similar to a 13.
I think that 3 second stages is over-kill for the no-deco recreational diver. Too much crap and too easy to get them confused. I use an Air 2 usually and have the pony reg around my neck on a bungie and the primary in my mouth. No regs clipped off and nothing to get confused. If I have to share air, I tell the buddy to take the primary from my mouth. He will do it anyway in an emergency.
I have no guage on the pony which I can read underwater, because it doesn't matter. If I have to switch to the pony I am comming up, who cares if it is at 1800 or 3000 psi, I am outta there.
I suspect that if I got a bad leak in my pony I would hear it and know during the dive that my redundancy would have become lost or compromised. I don't want to run another pressure guage hose for a pony, too much crap, too little benefit. I leave my pony on and I don't monitor it during a dive. I've never had a dangerous pony bottle leak in a thousand dives.
I have the pony behind me mounted on the primary tank because it is outta the way and I never use the damn thing. Mounting a pony as a sling bottle is not a bad option and has significant advantages with regard to entanglement and passing the pony off and maybe even ditching the entire scuba unit underwater if you were tangled really bad, however I think it is more important to have the pony bottle with you rather than arguing over how a diver chooses to mount it.
The arguments about taking doubles is crazy. Who wants to walk down a beach, climb over slippery rocks, scramble up onto ladders and moving boats wearing doubles if you don't need the air? A single tank of air will put me close to the deco limit, so why carry the extra volume of doubles?
An H-valve is an option, but it requires more responsibility on the divers part because he almost always will have to shut the problem reg down to realize the redundant benefit of an H-valve. If I have a bad failure, I can just come up on the pony and direct my attention toward controlling my bouyancy and breathing rate, not screwing with valves behind my neck.
I can see the benefit of the H-valve, but if you have 8 or 10 tanks like me, then it is cheaper to just buy 1 pony bottle and one pony regulator. Also if borrowing or renting tanks, H-valves are often not available.
