I am of course biased, but I would say join BA and join ASAP.
BA at your age offers you the ability to do almost anything you want - long haul, ultra long haul OR the, IMHO, far more enjoyable multi sector hopping around Europe stuff. Even within the short haul you can do trips or day trips, take an early command or be a safe pair of hands senior FO who does purely the nightstops that they enjoy and commutes from somewhere warm. To me it is a no brainer as pretty much no matter what you want, BA can offer it. The only caveat of course is being within striking distance of LHR…