Alien (demon) completely ***** on the final version. Passenger is good but what's the point of releasing such an amazing song when you're not going to give it the promo it deserves?
Alien is the best possible single choice from Britney Jean. Passenger would hypothetically be an amazing choice, but it would need heavy support and promo from Britney, which won't happen because she'll be in Vegas. She seems set not promoting the album because she wants to let the music speak for itself.
Alien would probably do the best without any promotion, as it's definitely the most acclaimed track on the entire album. That would sound amazing in a commercial.
I also think ISBE will be released after Alien: I hate that song but it seems to be the second most popular album track.