A Human sized target is easier to hit since it is larger. Besides that, it doesn't matter where you hit it, it will be going down, at "worst" to get cover.
That being said, targets usually don't move, humans do. It depends on the situation I guess.
I always was doing better at humans than at targets. We played around with our self made bows a lot when I was younger and I remember that I hit my friends much more often than a target.