I know it's a repeat/quote of what BusdriverLHR said but he summed things up succintly and accurately. I would take his advice if you're thinking of joining BA.
If you ultimately want to fly for BA, then my advise would be to take the first start date they offer you, regardless of fleet. If you aren't too fussed about BA, then you can afford to be a bit more picky.
Company or just another airline flying the aircraft you're rated on??
Which is it?