What it really comes down to is how much strength do you need to do a specific job. Balsa wood is a whole lot easier to work with, so I tend to use whenever possible and save the plywood for situations where you just can't use balsa such as firewalls and landing gear mounts etc. Foam core wings have benefits and issues as well, but the main thing to consider is you need the cores which most people do no have the equipment at home to generate therefore a built up wing is the way to go. If you look how enough planes are built, you will get a feel for what works and what does not. ARF type planes are very dependent on assembly line building techniques which may not be the best thing to look at when doing a home project. I would look at some plans and kits instead.