Multiple considerations would influence that. The BA215 is a 747 flight whilst the BA239 is a 777 so if you're in J and like the UD you'd probably want to take the 215. However if you're planning to work before you fly then the 239 is probably better timed for you. Of course, if you take the 215 you'll get to BOS in time to check in to a hotel & then go out for dinner, whilst on the 239 you'll arrive too late for that...