B6 is usually cheaper and is adding more flights later in the year from BOS to DCA but there are two issues with it:

1. B6 has no F cabin, all economy. Not sure whether you care or not.

2. Unlike AA, B6 has no interline agreements with any other air carrier nor do they have other connecting options. On AA for example, if BOS-DCA is not possible in a timely manner they can always reroute you through PHL, LGA, JFK, or CLT to get you to DCA. If they have nothing available on their end and it's a mechanical issue, they can even get you on Delta. B6 can't do any of that, it's either wait for the next available flight from them or rebook with a different airline at your own cost.