I second Ruth4325's advice above.
Personally I would always choose an HND departure, irrespective of the choice of plane -- convenience outweighs Dreamliner vs 777. In any event, NRT's 789 service has been known to be swapped out for a 777 service in the past.
Ultimately I would go with the most convenient airport given for where you're staying, and the most convenient flight time for your plans.