Personally I would go for the CO option via EDI.

If flying in economy any break in a trip is welcome just to stretch the legs and clear the mind. Best thing about it is, on your return across the pond you know you're landing at home and don't have to face connecting for the LHR domestic flight...always a killer in my past experience of returning to Scotland after a long flight across the Atlantic.
Having said all that it really depends on how convenient the EWR-SFO schedules are. If you have to wait many hours then you might as well forget the CO option.