Using an ATM card to make a fee-free withdrawal is almost always the cheapest way to get foreign currency. Period.

The only way buying forex from BoA before you depart is going to be cheaper is if you aren't able to find a Global Alliance ATM on arrival and end up using some other ATM that results in big fees. And even then it could still end up being a wash.

So if you can, get a Schwab card so you're not limited to the BoA partner ATMs. If you can't, then I'd probably still go the ATM route and just make one big withdrawal at the beginning for the cash I think I need for the whole trip.