Hey Darian,
You cant go wrong with either of these products. The choice to add a piece of foam core on the bottom of the box would be up to you, as I cant imagine an "imprint" would happen unless the stack of posters was left in the box that had a seam or ridge, for a VERY long period of time.. perhaps years.
If you are buying the acid free tissue, then any foam core (acid free or not) could be used as that bottom, protective sheet.
One example of acid free foam core available:
http://www.dickblick.com/products/elmers-acid-free-foamboard/