That foam you found is foam core, people build with it, but once you even think about touching the ground, it shatters and there is no putting it back together.
If you want to get really technical here, I am a beginner in flying. I have spent more time fixing and maintaining websites than I have flown (that is about to change). I do not have that much time under my belt on the sticks. Gaba, you have more time on the sticks than I do. But, I fly with balsa in my Edge. Never crashed. I almost did on final approach, but was able to pull out of it and land safely. Now, I have 3 more balsa planes (all 3D) and I plan on having the same luck with them.
The key here is patience. You let your friends either talk you into doing something above your skill level, or you wanted to show off for them and lawn darted that plane into the ground. When I fly, and I have flown with Wayne and Louis by my side talking to me, they never asked me to do anything "out of my range" nor did I try to show off. My landing sucked both times that I had someone there cause I was trying too hard to make it perfect, but they landed.
Here is the bottom line of it all. If you are wanting to go cheap, get foam. Like Travis said, unless you brace the crap out of it, it's going to flex and could break if you do a high "G" maneuver. Balsa is a lot stronger than foam, that is why you will see planes like mine doing snap rolls from 50mph, and never flex a wing. But, foam is cheap, that is it's point. But it looks cheap as well. I want smooth, I want to see lines from the balsa so I know it's quality. If we want a smooth rounded area, then we will sheet it with 1/16" ply (like on the turtle deck).
Either way you go, you have to watch out for hangar rash. If you bump a wing into something, I don't care if it's foam or balsa, it's going to leave a scar. If you grip balsa in the wrong area, you will snap a spar very easily.
I don't think I can explain it any better than that. If you want cheap, get foam. If you want quality that will last a long time, get balsa and ply.