I think foam is always going to be heavier than built up.

Whats all that space between the ribs in a built up wing? Air

Whats air weigh? Nothing

Whats foam weigh? Solid foam wing?

Even using 1 pound density foam--compared to 8 pound balsa. I still think balsa built up is lighter than foam. There is very little of that balsa in a built up wing.

Thats my totally biased opinion because I prefer a built up wing over foam sheeted. IMO--sheeting foam wings is a pain in the butt. I can frame up a wing pretty quick.