[soah]

I'd much rather raise the turn than bet the river. Betting the scare card on the turn usually means they didn't hit it, so the main possibilities are a pocket pair, or a flush draw. His bet is pretty weak though, and it looks more like an attempt to find out if his 99 is good than a serious attempt to move you off a hand. But I have no idea how players bet their hands at these stakes, so I might be way off.

Raising the turn pretty much commits you though, so you have to balance the extra EV of driving out a flush draw versus the lost EV of paying off with the worst hand. (Of course, if a worse hand will pay off to you then that should be factored in as well. Again, that's something I can't comment on at these stakes.)