Lots of threads on this in the past -- it is worth your time to do a search. I am firmly in the pony over SA crowd.
A pony is nothing more than a smaller sized tank. Instead of 80 cubic ft it might be 13 cubic ft or 19 cubic feet. (The original spare air was 1.7 cubic feet and their newer model is 3 cubic feet). Size should be dictated by your needs (air consumption, dive profiles, etc.).
You then attach a "regular" regulator (generally first and second stage with no Octo). You really need a pressure gauge -- I use a small "button" type gauge (others use a more traditional SPG, typically with a shorter hose).
So you now have your pony -- a tank, reg and SPG. The question is how best to attach it. First question: front or back.
For back mounting: The pros are it is out of the way, seems to work well with traditional bcs and there are lots of solid brackets being sold. The cons are that you can't see the bottle (and the button gauge), harder to control the valve and harder to pass of to someone.
Lots of options there. For years, I used something like the Ultimate Pony Bracket -- see
Ultimate Pony Bottle Bracket . There is also the pony tamer and x bracket (both appear to be solid products see
pony bottle bracket from ReefScuba.com ).
For front mounting: The pros are it is in front and can be easily monitored, shared, and manipulated. Probably less drag and all hoses can be tucked away. The cons are that it is sort of awkward at the surface (fine in the water), requires solid d-drings and it is a bit of a DIY solution. (it would look a bit like
Dive Rite Hose Retainer reviews and discounts, Dive Rite )
What would I buy?
a) Pick your pony size first. For tropical diving (given my SAC and profiles), I would be comfortable with something as small as 13. Locally, I think I would want a 19.
b) Decide how you are going to mount it. With a BP/W, I would sling it. With a traditional BC I would look at something that is a back mount (like the ultimate).
c) Decide on my SPG solution. In front, I would use either a button (or a regular SPG with a very very short hose).
d) Decide on what reg to get.