I think this is an easy question... which probably means I've got the wrong answer...

I vote for 12 because that means he's only getting 3:1 to call, and is 4.75:1 against (I think something like that...) to his flush on the river.

Betting the 30bb overbets the bot IMO, and at $10NL, he's more than likely to call regardless. The larger bet is very unlikely to increase my chance of pushing him off of the hand. So when the scare club hits on the river, I've got a bigger pot to bet into to test whether he was indeed on the flush draw and I'm going to end up losing more money.

So... 12bb takes the odds away. Means (assuming he only calls), that I can go with a 18-20bb river bet if the 3rd club hits and fold to a raise.