Well, it seems as though you've been offered three different options:
1. Dive a single tank with good buddies, and practice emergency procedures often. Personally, I think this is a very reasonable solution for most recreational diving, but it sounds as though that option is ruled out for you by the rules of the boats you go out on.
2. Carry a pony. The advantages are immediate self-rescue and some redundancy. The disadvantages are adding the complexity of a second tank and regulator, where the gas in the tank is not part of your gas planning for the dive. People raise various concerns about carrying pony bottles -- If you aren't careful with the placement of the pony reg, you can have a leak or freeflow without being aware of it, and find the bottle empty when you need its contents. If you include the gas in the pony as part of your safety reserve for you and your buddy (see "rock bottom"), then you have to deal with the question of which reg you donate. If you donate your primary but intend to put the OOG diver on your pony, you have to stabilize them and switch regs, which is a time of risk. If you donate the pony reg, you have to have it somewhere where you can easily reach it and it will reliably be there, but where it is very easy to get free to donate. Those two characteristics are somewhat incompatible.
If you only carry the pony for self-rescue, and have to carry rock bottom on your back, the pony isn't helping you much. And the wrong size pony (see the post above about 6 cu ft) will give you a false sense of security.
3. Manifolded doubles. This is the best way to provide redundancy and maximize the options for self rescue, but doubles are heavy and unwieldy. And unlike a pony bottle, where if you don't use it, you don't fill it, you have to refill doubles after use, and most places don't give you credit for the safety reserve gas that you didn't touch (cave country being an exception!)
Both a pony and doubles require two first stages; a pony setup may require three second stages, depending on your gas donation plan.
Although we do it, it is often difficult in diving to draw a line and say, "This is the ONLY valid approach to this problem." Each approach has its benefits and drawbacks, and each individual has to make his own decisions about what approach fits bets for him. What's important is actually to think about the pros and cons of each decision and get educated, which is what you did by asking the question here, which is great.
My strategy is single tank for shallower diving, with good buddies; doubles for deeper diving, regardless of buddies, and doubles for shallow dives with unknown or inexperienced buddies. But it's easy for me -- I don't do much diving off boats, and we own four sets of doubles
1. Dive a single tank with good buddies, and practice emergency procedures often. Personally, I think this is a very reasonable solution for most recreational diving, but it sounds as though that option is ruled out for you by the rules of the boats you go out on.
2. Carry a pony. The advantages are immediate self-rescue and some redundancy. The disadvantages are adding the complexity of a second tank and regulator, where the gas in the tank is not part of your gas planning for the dive. People raise various concerns about carrying pony bottles -- If you aren't careful with the placement of the pony reg, you can have a leak or freeflow without being aware of it, and find the bottle empty when you need its contents. If you include the gas in the pony as part of your safety reserve for you and your buddy (see "rock bottom"), then you have to deal with the question of which reg you donate. If you donate your primary but intend to put the OOG diver on your pony, you have to stabilize them and switch regs, which is a time of risk. If you donate the pony reg, you have to have it somewhere where you can easily reach it and it will reliably be there, but where it is very easy to get free to donate. Those two characteristics are somewhat incompatible.
If you only carry the pony for self-rescue, and have to carry rock bottom on your back, the pony isn't helping you much. And the wrong size pony (see the post above about 6 cu ft) will give you a false sense of security.
3. Manifolded doubles. This is the best way to provide redundancy and maximize the options for self rescue, but doubles are heavy and unwieldy. And unlike a pony bottle, where if you don't use it, you don't fill it, you have to refill doubles after use, and most places don't give you credit for the safety reserve gas that you didn't touch (cave country being an exception!)
Both a pony and doubles require two first stages; a pony setup may require three second stages, depending on your gas donation plan.
Although we do it, it is often difficult in diving to draw a line and say, "This is the ONLY valid approach to this problem." Each approach has its benefits and drawbacks, and each individual has to make his own decisions about what approach fits bets for him. What's important is actually to think about the pros and cons of each decision and get educated, which is what you did by asking the question here, which is great.
My strategy is single tank for shallower diving, with good buddies; doubles for deeper diving, regardless of buddies, and doubles for shallow dives with unknown or inexperienced buddies. But it's easy for me -- I don't do much diving off boats, and we own four sets of doubles
