Since you wrote about preferred seating I assume that you are looking for Economy class, right?

I would prefer SFO-LHR-TXL on BA over a red-eye transcon with a connection in ORD or JFK.
The SFO flight is quite nice on the A380 but currently B747s are used on both BA284 and BA286.
Rather than spending money on advance seat selection you could try to score an upgrade to BA World Traveler Plus (Premium Economy) while booking.