i would definitely go on a 747; first of all with 9 seats vs. 6 you will presumably have a higher chance of snagging the award seats. second on a 77W, because of the bassinet positions of 2A and 2K, you wouldn't even be able to reserve adjoining window seats (1A/2A or 1K/2K) that make the most sense for couples. whereas on the 747 you would be able to reserve well in advance multiple combinations of good adjoining seats for couples (1A/1K, 2A/2K, 1A/2A, 1K/2K etc.).

personally - on the pt of maintenance of 747s - if it goes tech at the airport I am sure CX would look after you quite well as an F pax. if in the air, and there is a tech stop, that would get you a lot more time in F then on the ORD route =)