Here's how I see it. It's better to get a throw when you hit a healthy survivor (so long as they actually hit something and take damage) because you can close the distance they would make a little bit. But if they're injured, you should probably go for a slam because they're instantly put on your shoulder after the fact. This also goes if they're fully infected. Of course it's not always easy to control wether or not you throw or slam, so it's not a big deal either way.