If Delta's PS seat is as good or better than the VS Premium seat, then the choice is clear, go with PS, particularly if the spread between coach and PS is $300 or less.
I fly from the DC area to London at least once per year, and for a 6.5 hour eastbound and 8 hour westbound trip, the Premium seat is a really good compromise, because you are getting what amounts to a domestic F seat, and the food is as good if not better than what Delta services in domestic F.
Delta C+ is coach plus about 3 extra inches of separation between rows.