Ok, here are your worms (you DID open the can!)
I now dive with a pony. Why? I spearfish. We often make poor buddies, and I've both gotten separated before (unintentionally) and also INTENTIONALLY dove with someone when we were both "on the hunt" and, while within visual range of one another, not really what the DIRites would call a "buddy team".
At 60', this is a problem, but not a huge one. You might get hurt doing a CESA from 60-70', but you'll probably also make it.
The problem is that we often dive at 80, 90, 100 or even 110', and I'm not AT ALL sure I can make a CESA from 110', nor am I going to attempt it to see if I can indeed do it without drowning!
The next question that arises is "do I do doubles or a pony"? For me, right now, the answer was "a pony." Why? Because doubles on a boat are a heavy biatch to deal with, I don't need the gas capacity they offer, and while there are advantages to doubles, what I'm really after is redundancy - which is actually LESS present with manifolded doubles than a fully independant "spare" kit in front of you!
Ok, so I made the decision I needed a pony. Now the next questions are "what size", and "how do I carry it?"
The "what size" question was easy to answer. I needed something that would permit both a REAL and a SAFE abort, AND it had to be useful at depths up to 130'. This, for me anyway, meant a 19 cf bottle. I can do a full ascent from 130' on this, make my 3 minute stop at 15', and have gas left. If I start at lesser depth, I can even do light deco (a few minutes) and not run out of gas. I fill the pony with AIR, not Nitrox, because I do not want to worry about a hard floor AND I want to "practice" using it regularly, which means it will frequently get topped off - dumping mix is wasteful. Finally, there is no real impact on NDLs in this case, since if you draw it the dive is over and you're ascending.
The final question is "how carried?" I fought with this. Most of my dive partners carry theirs strapped to their primary tank. I decided to sling mine, for a number of reasons. In no particular order:
1. Its out of the way. You wouldn't think so, but it is. The bottom rides up and it tucks under your shoulder; in the water you don't even know its there. On land its a PITA, but that's not where you use it, right? A giant-stride entry is pretty easy; I "press" it against my side with my left arm; the same hand holds my reg and mask front in on entry; no problem there. I turn the bottle on, charge the hose, then turn it OFF - this prevents a freeflow problem if the purge gets hit accidentally; the valve is immediately accessible (literally "in your face") if you need it.
2. I can read the gauge. I use a "stage-style" rigging on a 6" HP hose and a regular gauge bent back and secured to the first stage with a piece of cave line. Its "right there".
3. I can hand off the ENTIRE THING if someone ELSE needs it. Two bolt snaps are all that holds it to me; I can thus give it to someone if they need it underwater. The reg is on a standard octo-length hose.
4. If it gets hung up on something, I can see it to free it. Not possible if you get hung up on a back-mount.
5. It doesn't seem to affect my trim at all. I like that.
6. Finally, in a TRUE emergency, where I'm entangled behind myself and can't get my kit free, if I am personally not entangled I can ditch my entire KIT and ascend on the pony! Yes, that will be a true "brown water" scenario, but having something to breathe in that case beats the hell out of not!
