I use a around 2 per G myself, 5-6" Grade A Bourbon Madagascar. I soak in either high quality bourbon or vodka for a couple weeks beforehand, add the whole thing. Last batch i did they soaked in 6 ounces of Stagg Jr. bourbon whiskey, before that I used EH Taylor Barrel Proof. I prefer using high proof bourbon whiskies, but I've had good luck with vodka for a much cleaner vanilla profile without the Bourbon whiskey influence.
That said, Bourbon vanilla beans are my preference, but so are tahitian and mexican. but you should really try each to determine what flavor profile you want, each vanilla bean is notably different. I'd like to do an IPA with Tahitian beans some day soon.