The primary reason for me to consider the pony is that it's my understanding that it's required by some, not all, dive operators for some of the deeper dives, within rec limits, in the northeast. While I may be wrong, this is my understanding.
I was told that I need one for one of the dives around here that I'd like to do this summer, and I was on a dive boat off of New Jersey last summer where they'll only let the divers below a certain depth if they have doubles or a pony. I can't remember what that depth was, but my memory tells me it was around 80 feet. Maybe that's good safety and prudence, or maybe it's a way to make extra money by renting ponies.
(As a side note, it didn't matter that day, anyway, because the dives were only in the 50-60 foot range. Never mind that the anchor came loose and I surfaced to no boat because of it....)
My thought has been to mount the pony to a bracket on my tank.
All of my diving has been, and my future dives plans at this point in time remain, to stay within rec limits. So, I'm not planning on the pony for expected deco purposes, or any other planned needs.
I'm thinking about having the pony because it might be required if I want to do certain dives, and because it would be nice to have the extra gas for unplanned needs. My dive plans do not require the extra air. It's for something unplanned. My thinking was that I should not be in a situation where the extra gas is necessary. But it would be even worse to be in that situation and not have the extra gas.
UP, I had ruled out a larger tank for 2 main reasons: 1) a bigger tank still means no pony which might result in a problem if doubles or a pony are required by a dive operator; and 2) the negotiation psychology with my wife of buying a "small extra bottle of air for emergencies" versus buying a new big steel tank....
Thank you, everyone, for the thoughts so far, and please keep them coming. I appreciate it.
Michael