If you carry two tubes you're guaranteed to never need them. If you only carry one there's a good chance you'll need the 2nd. Commuting, I always carry two tubes, a CO2 and a pump. I usually use the pump when I'm commuting. If I'm on a group ride I'll use the CO2 for speed.

edit: I wouldn't change tire types based on a single flat. Flat's are far to random to make decisions on individual events.