I disagree. Airfoils tend to fly smoother, but are designed to fly, not be stalled. I find that airfoils rock more in harriers than do flat plates, and generally don't 3D as well. Flat foam, however, is stalled most of the time it is flying, whenever the AOA is greater than about 7* on average. However, flat plates tend to like the stalled wing high alpha 3D manuevers better, in my experience.

Now, to really compare foam vs. balsa, if you had two of the same airframe, one balsa, one foam, both with airfoils, same design, I would think the balsa one would fly better. It's more rigid, for one thing, and less draggy, as the covering creates less friction than does the foam. However, the foam one would be more durable, and easier to repair if crashed.