Craig, both pattern and penetration are important. The more pellets that strike a bird, the better the chance that one or more of them will hit something vital. And smaller shot gives you more pellets per shot charge. On the other hand, if none of those small pellets penetrate far enough to hit anything vital, the bird in question may very well keep on flying. I've seen pheasants fly off after shedding a pillow's worth of feathers. That's why you need penetration . . . which is a commercial for larger pellets.

Debates about the right combination of pattern and penetration can go on forever. I think some have!