I am faced frequently with similar dilemmas, and I prefer to take the shorter flights in F on AA, than the B6 longer flights with "even more space". Because there is no much price difference, I recommend DL in F, if that's still a choice. B6 is likely a 3-3 seat configuration and a long flight (if it was 2-2 it would be better). I am mostly Covid aversive and I mask, and still prefer shorter flights in F. But even an N95 may not handle a Covid risk for 5 hours in a crammed space. My comfort in economy would be 4 hours max (masked), but I would still do 5 hours if needed.