Things to consider, followed by my thoughts:
Seniority (date of joining) - The more senior, the better.
Aircraft type - Boeing is in my opinion, far more fun to fly than Airbus, and the 787 is the most modern Boeing.
Fleet route network - 787 is a bigger fleet and therefore will have more variety than the A350.
Future fleet expansion - I think BA is more likely to expand the 787 fleet, and indeed already has expansion hulls, they’re just grounded due Rolls Royce problems.
Future career in BA - If you don’t do Boeing now, when will you? Do you want to limit yourself just to Airbus for the rest of your career?
When I joined BA, I had the option to join when I did, or join a year later. I decided to cut short my plans at the time and join the year earlier. To give an example of how that has benefitted me personally: Had I waited the extra year, I would not have gained my long haul command when I did, 19 years after joining, (which was about one year before Covid hit). Instead I’d have been coming up for long haul command just as Covid popped up, so that would have delayed me by several more years. You never know when the music might suddenly stop during your career, but one thing for sure - additional seniority always helps.