Pony bottles and primary donation stem from two fundamentally different areas of diving, so you tend not to see the two discussed together, but I'll lay it out for you.
Primary donation is better, just is. It's in your mouth, it's always there, it's known to be working, etc etc. Don't buy into the golden triangle BS because it is just that, BS. Having the secondary on a suicide strap/necklace is just a superior way to dive.
Now, pony bottles are an interesting beast because you have to ways to choose to use them. Your backup gas, or your buddies backup gas. How you make this decision will determine your setup. If you choose to have it as your backup gas, then you would still donate your primary and instead of or in addition to having a secondary second stage, you would then switch to your pony. In this situation the pony is ideally attached to your primary bottle and you would use it in place of your secondary second stage. This requires a sufficiently sized bottle however so you have to do rock bottom calculations to make sure it is big enough. For recreational diving, an AL30 is about as small as you should go. This will pretty safely get you up from 130ft. You can get away with an AL19, but they aren't particularly useful for anything else where the al30 can at least be used for decompression later down the road.
If you choose to donate the pony bottle *not my recommendation*, then you can choose to do that, but the downside to this system is you have to sling the bottle and your buddy has to be able to take it. This system is not compatible with most traditional BCD's so I would not set my system up to have this as the primary method. It works fine with a team full of bp/w's because of the d-rings, but a traditional jacket doesn't have the ability to manage a slung pony easily and most recreational divers will not be comfortable enough to manage a bottle transfer. Combine this with the tendency for panicked divers to grab the second stage out of your mouth, and you may as well plan for the worst and have your primary be ready for donation. This also gives the likely panicked OOA diver the most amount of available gas since it's in your primary.
The issue with pony's is they aren't always available if you travel, so for local diving they're fine, but if you own your own tanks, I would go to an H-valve instead which still gives you regulator redundancy but isn't a lot of extra gear to have to deal with. For travel you can change your reg configuration and dive a normal single tank setup. The other advantage to H-valves is if doubles are intriguing to you, it will be an easier transition to that type of configuration and the regulators won't require any changing.
