The 239 is a late evening westbound departure. These flights in my experience can work well provided that you do not sleep on board. You can head straight to bed when you get to the hotel. I think it is a good choice to take after a day's work in London. (note: my experience based on evening middle east to London and far east to middle east departures at similar times with similar time differences)

The 238 is an early morning eastbound departure. It is brilliant for avoiding tiredness because you do not need to sleep on the plane. Furthermore because you have only a short time away it will not be a problem to wake up early enough to take it. A 5am start in BOS is the same as a 10am start in LON. I think it is a good choice. (note: my experience based on similarly timed and geographically close IAD-LHR)