Which class?
I think I'd prefer London. Longer flight to get more sleep.
But beware (and this isn't the normal LHR scare story). BA are moving all sorts of flights around during March/April. The Houstons are moving to Heathrow and I think they will move to T5 from end March. However, not all the T4 flights are moving until end April. I think the domestics also move to T5 end March but make sure you know which airport is being used *and* which terminal.
The class issue is that CO are 2 class while BA is 4 class. Depending on your budget etc you may get a "better" (subjective) seat on BA by paying a little more (although domestic is one class).
Oh, and check the seating on the CO 757 EWR to EDI if you're travelling business/club. Just be sure you're getting their full international business class.