I was planning on only using carbon tube spars and some monokote. I want to try and get maximum crash survivability with the simplest repair (or replacement) methods. Basically, I'm trying to make a good wing with the least amount of cost and labor. I imagine a balsa covered foam core would look great though, and be quite strong. And it might not be that difficult to repair, I'm just not familiar with the methods. There's always more to learn in this hobby.
If you want to maximize survivability then don't build it stiff (carbon, foam, etc...) EPP foam cover with colour tapes is the best. Unless you want to mould the whole thing out of carbon fibre fabric then that will be strong but still will crack on hard crashes, and expensive to buy carbon fabric.