cores: foam vs wood?
Balsa cores offer the best combination of structural properties per pound and was originally considered to be most durable of the coring choices.
The newer medium density closed cell foam cores (since 1985 or so) are a really wonderful solution but come at a weight penalty to Balsa.
Plywood cores are strictly a cost cutting measure normally in areas where labor is cheap. It gets widespread use on teak decked boats because it holds screws better than other cores.