RE: 20 or 12 guage?
If it was me I would go with the 20ga. You will have less recoil with the 20, plus more accuracy, and range shooting slugs.If you want to reach out there a long ways to shoot turkeys just get you a good choke, and some Nitro's, or to a lesser extent Hevi-13.
Good luck, glad to see another kid getting involved in the outdoors.