The decision on building materials should be made based on the plans and expected performance. If you're building a small electric, unsheeted foam can be a great, lightweight building material. If you're looking for more speed, additional strength might be needed, which might require balsa or fiberglass covering over a foam core. Balsa is a simple material to work with, but will require some work. Remember, all decisions are a tradeoff. Lighter building techniques often sacrifice strenghth (OK, not if we're talking carbon fiber).

I just finished a plane of my own design that is a foam core fuselage and foam core wing, with 1/16" balsa sheeting. 68" wing span, 54" fuselage length, final weight about 8 lbs. I could have built lighter, but its quite strong. I like the way it flys, but others might not. I may try building the same design next winter with a balsa stick frame.

Let us know more about your expectations for the final product, and then we can advise on materials.

Brad