Lots of things to consider:

1. Who will pay for the tickets, you or your employer?
2. Do you expect to check a bag on your flights, or carry-on only?
3. Where would you like to travel with your miles, and in what class of service?
4. How important to you is having lounge access in BOS/WAS?

Other things to keep in mind:

1. AA has interline agreements with DL and UA, so in the event of IRROPS (irregular operations), AA could theoretically rebook you on either of those carriers at no cost to you. B6 does not have such agreements with AA, DL, or UA.

2. Just because you fly on a particular airline does not necessarily mean you have to credit your flights to that carrier's frequent-flyer program. If, for example, lounge access is important to you, earning elite status with one of AA's oneworld partners might make sense. (When you and your fiancé travel together on the same booking, you will enjoy your fiancé's status benefits when flying on AA or its oneworld partners.)