Tanks:
My wife used a 15 cu ft Catalina. The advantage was that the service pressure was 2015 psi and you could transfill it from an AL80 at the dive site. Plus the bouyancy traits of both the 15 cu ft Catalina and 14 cu ft Luxfer low pressure tanks were great. They were esseatially neutral when empty with the valve ad reg attached (so they required no additional weight to be used) and were only about 1.5 pounds negative when full with valve and reg attached, so they did not massively overweight the diver.
The currently available 3000 psi ponies from both sources are couple pounds more negative than their previsouly available low pressure counterparts so you end up having to remove a couple pounds of lead to remain properly weighted.
OMS offers 2400 psi ponies but they are again a bit too negatively bouyant for my preferences.
I would go with a 19 cu ft tank as compared to a 13 cu ft tank. It will be about 5 inches longer (which make no difference in how the tank rides or feels underwater regardless of whether you tank mount it or sling it.) A 19 cu ft tank will only be 2 lbs heavier (6 versus 8 pounds) and the diameters are identical. And the larger 19 cu ft tanks are normally about a pound less negatively bouyant.
The Luxfer and Catalina 19 cu ft ponies are virtually the same bouyancy wise with the Luxfer being slightly heavier in weight (about 4 oz), an inch longer and with a capacity of 19.9 cu ft as opposed to the Catalina's 19.0 cu ft.
In comparison a Luxfer 30 is about 3.5 lbs heavier, 3" longer, and .5" larger in diameter. If you don't mind the extra weight, it offers 50% more air and is large enought to make a useful deco bottle which can help the resale value if you sell it. The bouyancy traits on the Luxfer 30 are great in that they are back to being neutral when empty with a reg attached - a very nice trait for a deco or pony bottle. The Catalina 30 is less desireable as it is 2 lbs heavier and another 1/2 inch larger in diameter, and is another pound more negatively bouyant.
In short, if weight is a factor go with a Luxfer or Catalina 19, if 3.5 lbs won't cause problems, go with a Luxfer 30.
Regulator:
Regulator wise I am a big fan of simple unbalanced piston designs for pony bottles and deco bottles. I currently use Scubapro Mk 3's and Mk 200's on mine - including the original MK 3 I bought as my first regulator in 1983. They are extremely durable, very simple and very reliable. The design is great for emergancy use as it uses no high pressure o-rings in dynamic applications and has only 2 dynamic o-rings in the entire system.
Another nice thing about an unbalanced first stage is that even with a balanced second stage, the inhalation resistance increases noticeably when the tank pressure falls below about 300 psi, so you get ample warning that you are alomost out of gas - this allows you to maximize a safety stop without running the risk of having to do an out of air ascent from your safety stop even if you are not using an SPG.
The current Scubapro R190 and R295 second stage that are packaged with the MK 2 are fine for pony bottle use and will deliver good perfromace to 130 ft. For deco applications I prefer balanced second stages (given that on a deco dive you spend more time on the deco regs than on your primary and a very easy breathing rgulator is very nice to have.) I used older metal cased Scubapro Balanced Adjustables since they also have virtually no problems with dry mouth (which can be a problem if breathing aviation grade O2) but the new G250V with it's metal air barrel or the original old style G250 would also be a great choice and an SP dealer can order a MK 2 G250V combination from Scubapro.
There is the option to buy good used equipment and I am of the opinion that a properly maintained and serviced used regulator is often a much better inverstment than new equipment.
SPG:
There are three options.
1. No SPG - The logic here is that a pony is for emergency use only and should be checked at the surface prior to each dive. Since you know it is full and you will be ending the dive and immediately ascending to the surface, underwater knowledge of the contents of the pony is irrelevant. The downside is that you need to check the air on the surface which means having a dedicated tank pressure gauge or alternatively hooking up your primary system prior to each dive - which over the course of numerous dives can use a fair amount of air limiting what you will have if/when you need it (another argument for a 19 versus a 13).
2. A normal SPG on a short 6 or 7" hose, if carried as a slung pony or on a normal lenght hose if tank mounted. - This allows you to readily see the remaining tank pressure under water but adds an additional hose and additional mechanical complexity in the form of another high pressure o-ring and the high pressure spool.
3. A button SPG on the first stage. - This allows easy (if not exactly plus or minus 100 psi precision) confirmation of the tank pressure prior to the dive with no gas loss and with reduced complexity (no hose and no HP spool to fail and leak) underwater. It is a bit hard to read underwater on a slung pony and is impossible to read on a tank mounted pony. But it does offer a reasonable compromise between a full blown SPG and no SPG and the cost is inexpensive at about $35-$40.
DIN or Yoke connection:
My preference is DIN regardless of whether the pony is tank mounted or back mounted as it is both more secure and less prone to snagging on line, etc. Go with a 232 BAR convertible DIN/K valve and keep the yoke insert in the valve when you travel to ensure you can get it filled at your destination.
