First, you need to tell us what type of flying you will do in addition to the xONEx if any. Are you after miles or status? Do you want to fly BA USA-LHR? Each one has its own advantages (sorry about the pun) but advice is limited until more is known. As for seats, it is usually more difficult to get 2 A class seats (on long-hauls mostly), since there are only 8-12 seats per plane. D class is easier as long as one is flexible with schedules, though D class may be limited for some specific flights at specific times of the year (think holidays).