Take the 787. Six weeks could make a hell of a difference to your first LH command in the long run. An Airbus to Boeing conversion is way easier than the other way round and the number of people failing any conversion for that matter is vanishingly small. The A350 is a small fleet and will remain so (I think the future orders and exercise of options now rightly or wrongly will be between the 787 and 777-9 - they haven’t decided to switch to GE engines for a sub fleet of 6) so your relative seniority isn’t going to move as fast on the 350 as on the 787. More variety on the fleet too.