[microbet]

I think a weak ace is betting the river more than he's calling a raise on the turn.

If he's very loose and would have called a raise on the turn, and he bets the river, he'll call a river raise with any hand that would have called a turn raise, but he's not going to call a turn raise and still lead the river.

He'll take a small shot or something with air on the river a reasonable amount of the time.

A pair weaker than an ace will call a river bet a lot more than a turn raise.

I don't think you miss any value here by calling behind even if he has a hand as strong a A9.

If the lines were exactly equivalent in terms of value, then I'd go with raising turn because he still could have a draw, but I think his chances of bluffing river + chances of leading with a weak hand + chances of leading and then calling a raise on the river with a medium hand absolutely prove that I like to repeat myself.