What airline(s) have you actually flown?

If you've not flown either AA or Southwest, do you know whether you will want to fly them?

For example, assuming you have no status (though you could get first level status if you fly BOS<->California decently frequently with the same airline each time), you'll have no assigned seating on Southwest (but ability to choose your seat from the remaining ones as you board), while you'll have forced assigned seating on AA (often no ability to select anything other than middle seat without paying extra, until day of check-in). I thus don't know if you should pick an airline solely by picking its credit card first.

On the other hand, once you have elite status at AA (after 5 paid round trips BOS<->LAX in the same calendar year, for example), you'll have much better selection of seats, including half-price on Main Cabin Extra (extra legroom seats) in advance and free Main Cabin Extra at check-in, and you'll occasionally even be able to get an upgrade. While flying 5 paid round trips BOS<->LAX on Southwest won't change your flight experience much.

The status works a little differently at UA, because UA requires that you spend $2500 on those paid trips (not just that you fly 25000 miles), so it depends what sort of fares you're geting on your BOS<->California trips whether UA would get you to first-level status as quickly as AA would (AA at least for the next year doesn'te care what you pay, just that you fly 25000 miles).

But 5 BOS<->LAX trips is just barely over 25000 miles. So if you only do 4, you won't get any status.