Of course in a real fight you want to avoid getting hurt -- that's your number one priority. But I would think that you also want to avoid hurting the other guy as much as possible (because it's the humane thing and also because you want to avoid legal trouble).
So really you want to get to a position where you can CONTROL the other guy without hurting him. And the position has to be one where he can't hurt you and one where you can maintain the position easily and without much chance of getting surprised by a "dirty" tactic.
So what positions do you want to achieve? Here are my thoughts:
knee-on-belly, protecting your groin. A good position to control and avoid getting too close to a headbutt or a groin grab. A good knee-on-belly takes the fight out of the other guy pretty good because of the pressure on his diaphragm and abdomen. (I'm sure you've experienced this in class). You can also just get up and run away really easily.
rear bearhug. You need good head position so he can't snap his head back and headbutt. You also need to watch your angle so he can't attack your groin or stomp your feet. I don't think a harness grip ala Marcelo Garcia would be ideal, because there's a chance he could bite your top arm, but if you could get your arm under his chin quick enough,
the RNC is king. The only problem is that this is a very scary position for laypersons and I'm not sure how favorably a court (or jury) would see this position. For example, a layperson might see this as a "killing" hold and I could see attempted murder charges arising from this and witnesses who could convincingly testify that you were trying to kill the guy. Even if you were to just hold the guy in place and don't attempt to choke him, it still LOOKS like you're choking him. BIG problem.
-face down back mount with him stretched out. you can control here pretty easily, it's pretty obvious that you're not hurting the guy -- just kinda sitting on him, to a layperson -- and it takes the fight out of the other guy.