if one was going to put down plywood on a wood plank type of sub floor would you screw it or nail it down? thanks

After you cover the floor with the new plywood, you will have trouble finding the floor joists to fasten it well. I would use hand driven nails so you know if you hit a joist when you drive the nail through the plywood and have fasteners penetrate into the floor joists.
If you do use screws, use some with a smooth shank that doesn't have threads the thickness of the plywood, unless you pre-drill each screw hole the diameter of the threads, through the plywood so the screws will pull the plywood tight.
The nailing schedule for underlayment is nailing every 8 inches around the perimeter of each piece and every 12 inches in the field. If you don't hit something and go between the sub-floor planks you may get squeaks as a nail slides against the wood even a tiny bit.
Do you know that solid planks are stiffer than plywood? Make sure that the plank sub-floor is nailed, or fastened well, before the extra plywood goes down. If you are using 2 inch staples and the Bostich M III stapler for fastening the flooring, you can take the nose off and use it to fasten everything down.

Thank you very much . Not sure about the stapler. What else can be used to make sure sub floor is tightly secured in any areas that may be loose? Screws or nails?

If you use screws which hold better than nails, you may want to pre-drill for each screw only through the plywood, so you can tell that you are screwing into something solid. This will help with countersinking the screw heads, too. It will take longer, but you can be sure that the ply and boards are fastened together.
If you use screws that have a smooth shank near the head and threads below, the smooth part will allow the screws to pull the pieces together better than threads all the way along the shank, unless you pre-drill.

Thank you.