I'm just going to answer the question you actually asked.
I have another thread on a dive i will be doing this weekend. The pony tank that I want to use is a brand new 30cf and I will be using an aqualung core supreme reg on it. Not that these things probably matter but I have to buy a mount/harness for the pony. I know that this is purely a matter of preference but I am wondering if YOU THINK it is better for me to sling this or to strap it to my primary tank.
There are a number of problems with back mounted ("strap it to my primary tank") ponies.
1) Generally, it is not possible to reach the valve, so the valve must be left open for the entire dive. There is some risk of loss of gas due to a freeflow or leak, and this loss going unnoticed. There is also a risk of the valve being left closed inadvertently, rendering the pony useless.
2) It is easier to mix up the regulators, possibly with grave consequences. This is especially true if the pony is mounted valve-up, because then the hose routing will be all but indistinguishable from the primary cylinder.
3) It isn't possible to see a button gauge mounted on the first stage, so there's no way to visually check the pressure during the dive.
Now, there are mitigations for all these, but they pose some of their own problems. Some people use inline valves right next to the regulator to prevent a freeflow (but it's one more part prone to failure, and one more procedural piece). You can color code the regs or hoses to reduce the chances of a mixup (but in low viz or at depth, in an emergency, you might not be able to tell the difference). You can use mouthpieces with a different feel. You can do things like always use the pony reg with a neck bungee so it's in a particular place.
The only real problems with a slung pony are: 1) that you have to have appropriate D rings on your BC to attach it, and 2) it is more likely to get in the way, particularly if you're engaging in underwater photography.
do I still use my octo for the primary tank when using a pony? That would be 3 regulators :/ Thanks for all the help!
There are several alternatives here.
In general, people using pony tanks have 3 regulators. That way you don't have to change your basic regulator configuration around when adding or subtracting the pony, and you are able to share your full compliment of back gas with an OOA diver.
Some people only use two regulators, either because they dive solo and don't expect to encounter other divers, or because they are depending on putting one diver on the pony and the other on back gas in the event of an OOA emergency.
With a slung pony, you can dive any reg configuration that makes sense for you for your primary cylinder, and the pony just adds to it. This is what I do, using a 7' hose, a bungeed secondary, plus the pony.
DumpsterDiver, who I believe is banned at SB now, had extensive dive history with a back mounted pony, and was probably the cheif proponent on SB of this configuration. He used an Air2 (a combination BC power inflator and backup regulator) on his primary cylinder, and a bungeed regulator under his chin for the pony. I think that's probably the best way to set up a back-mounted pony in that it results in minimal gear and its hard to mix up the regulators. There is still some risk as you can be on a neck-bungeed regulator without realizing it.
