Starting in October, SAN will be serviced by a mid-J 747, so no chance of WiFi and there is also the chance of an unrefurbed airframe. LAX has the choice of an A380 or a very high chance of a Hi-J 747 which have all been refurbed. (I believe the 789 service has been replaced by a 747 these days). The SAN flight will also arrive at LHR T3 from October, so you might have to take that into account if you have a connection to make at LHR.
Despite living in San Diego, I actually prefer to go out of LAX as the early LAX-LHR departure gets me into LHR at a reasonable time in the morning, vs. a late afternoon arrival if on the SAN service.