I just got back from turkey hunting, my favorite hunting, I think.
If you have a 3 1/2" gun, I would pick the Fiocchi nickel plated loads containing 2 3/8 oz. of #4 shot.
For a 3" gun, I would take any purposely designed turkey shell with lead shot, my experience with Federal Premiums leaving no room to complain.
My all time favorite shell was the Active Penetrator, with 2 1/4 oz. of #4 shot in a 3" shell. Unfortunately, they are no longer available, although their hull design was superior to most others.
A lot of people swear by #6 shot, since they hope to be shooting at under 30 yards. I don't know of a #4 load that doesn't shred targets at that range, so I will take the bigger size shot and move my effective range out another 5-10 yards.
You might want to consider backing up your first shot with a Hevi-shot in #2 size shot. What would normally be the easiest shot in all of shotgun hunting can be complicated by shooting from awkward positions, nerves, and brush, so if something goes awry, it doesn't hurt to have a load that can snap a wing bone or drive through a bird from behind.
This advice on the #2 Hevi-shot only goes for birds that have been hit, and you are trying to bring them to bag. I used to have a primary flight feather from a gobbler that had a #2 copper plated shot from a Winchester Super Double XX embedded in the quill. A turkey that is standing or running broadsidehas his wings folded over his body, meaning you have to shoot through three layers of feathers before you even break the skin. If you can't shoot at the head/neck on the first shot, don't even shoot. I would rather take my chances with a cheap promotional #6 shell aimed at the head than with any super shell at the body.
I will post a picture of the birds soon, hopefully.