Depends on your market. Posterboard is more portable, but it puddles easier. Canvas is a little more forgiving, but harder to transport. If you're just trying to get your art out there, Posterboard is quicker. Or if you're doing conventions or Street art, Posterboard. But the paint doesn't absorb into it. So your technique has to be a little more refined, if only so you don't have gloopy paint splotches.