Weight is the main factor. If it never moves, that strikes a blow for the foam. If it's portable or needs to stash away, then foam is easier to handle but not very durable (corners, etc.)
Sound - both are loud. Plywood normally requires something added on top (homasote, foam, cork) and foam as a table top isn't as quiet as some might think.
Holding nails or screws - Plywood for sure. Foam might hold for a while but the material is soft and those holes will eventually enlarge through movement.
Carving terrain - both need to have foam added in layers to build upward. To carve downward, the foam is easier but messier.
