I was in a similar situation to you a few years ago and was fortunate to be offered a job. However, reality gradually dawned that I would eventually get bored doing long haul with 10 years+ to a command that I already had in the short haul world.
It is without doubt a good company with plenty of aircraft types and opportunities but you will be in the bottom half of seniority lists for a large percentage of your time there. It is a tough choice but what finnally swung it for me was doing the pension sums using X years at BA as Fo/Capt and X years at my current job as a Capt. The figures will only ever be approximate but will help with your decision.