the problem w/ the 99 hand is that the board reads Q-2-3 5. Because there was a flush draw on the flop I would be more likely to bet the turn than otherwise, but the thing is, on Q-2-3 what can he really call you with on the flop and the turn? If he thinks you are super aggro I guess he could call w/ a 44-88 type hand, or 5x. I would much prefer this if the board was a little more draw heavy and I knew my opponent to call too much w/ draws and never play them fast. And/or if he thought I was just out of my mind I would definitely fire here and not be extremely concerned about the Q. It's a feel type thing, either way, when the Q pairs the river your hand improves, as does his, plus it's less likely he has a Q. Though it is more likely for him to think you have a Q if you bet, which means you likely won't get paid off by worse unless again, he thinks you are totally insane, then he will be more justified in calling you down very light assuming you are bluffing. I'm not betting this river unless one of those dynamics I just described existed, if not, c/f.