It's true that I don't *really* know villain is a nit. He could be a nit, he could be a tight player who has been a bit card dead, or he could even be a reg type with an average range who has been card dead too. But if I waited to have completely solid information on every player's style before using that info in games, I'd be throwing money away. The best I can do is play against this guy like he's a nit, and be prepared to reevaluate if he does something strange for the sort of player I think he is.
I am starting to think this is just a close choice and both options are fine. We're shallow enough that by 3-betting, we make more off AK than we lose off getting stacked with an overpair. We are in trouble if he is really flatting with [AA-QQ] or [AA-QQ,AK] but I would expect jacks to come along most of the time. Maybe that's wrong, I'm not sure.
But if we just flat, we have more room postflop and we might get people to call with hands way behind both me and villain. Shortstackers may call with hands like 88 which are not getting nearly the equity they need. The table is tight but I might still get some action.
Follow-up question:
If the flop comes down with an ace, my plan is something like:
Flop ($100) : A
9
3
Villain checks, Hero checks
Turn ($100) : 4
Villain bets $60, Hero folds
right? Since I'm getting crushed by [JJ+,AK] once an ace or king falls, and a nit is unlikely to bet much with a hand I beat.