You said this would be your only double. That is what I thought when I bought my first. If you are under retirement age and find you really like doubles you will find a way to scrape up enough to buy another one or trade up to a different one so don't stress on this decision too much. You may find you don't like doubles. Many who buy one find they are not for them especially doubles in the harder recoiling calibers. 9.3 is quite popular for this type of rifle but in a light double they do kick much harder than a 12 guage or 30-06 and ammo costs a lot more than for the 30-06.

A low pressure rimmed cartridge like the 9.3 is preferred for a double simply because extraction is theoretically easier and therefore more reliable. This may be important when hunting dangerous game but doesn't matter for anything in North America except brown or polar bears. I own doubles in both 9.3 and 30-06 (as well as .470) and like all three. If money is really tight the cheaper 30-06 ammo may help make the decision.

A 30-06 will kill any pig, a 9.3 just drops them a little faster-only however if you hit them, and hit them in the right place. The flinch you get from shooting a hard recoiling caliber you don't handle well can keep you from hitting them in the right place. Try to find someone with a 9.3,.375 H&H, OR .375 Ruger (not a .375 Win) that weighs the about same as the rifles you are considering and shoot one from several positions to see if the recoil is an issue for you. If so go with the 30-06. If it doesn't bother you get a 9.3. Good Luck!