You seem to be asking about getting through security, since you mention BSB and ATL airports, but don't mention Customs. I assume you've already checked on limits for bringing liquor through U.S. Customs (from any country). Nothing Brazil-specific as far as I know. Going through airports is a matter of security: she could get through security with the bottle in her carry-on bag flying domestically from BSB, but as soon as she is boarding an internatiional flight to the U.S., nope. Same rules as in the U.S. (though I don't think she has to remove her shoes). So, as suggested above, it would have to be in her checked luggage (if she were to buy liquor in duty free, at BSB or on board her flight to the U.S., she'd have to put it in her checked bags before going through TSA security if she changes planes at ATL-- my wife forgot about some face creme she bought in duty free and lost it going through security at ATL since we'd already passed on our checked bags at the transit desk).

Which cachaça to buy is very individual. Some buy for a nice bottle, especially if it's a gift. I am in Brazil and don't know of any really smooth cachaça (it is sort of a rough-edged cousin to rum, which is produced in small quantities in Brazil). It's best for mixing caparinhas, etc. In Fortaleza, where we are, Ypioca is most popular. If you're going to use it in mixed drinks, I'd suggest she buy whatever's cheapest or comes in the nicest bottle or conveys memories for her. Few if any would likely detect the difference between any cachaça used unless it's exceptionally harsh (as in a moonshine-type) or is flavored. Some brands come in bottles wraped in dried leaves, or woven-covering. I do hope she saw more of Brazil than Brasilia.