A few comments:

If you want to go on the UD you can pay to reserve your seats. As a couple a pair on the LD might be prefered anyway. Or why no try one deck on the inbound and one on the outbound so you can compare. If one of you is over 6 feet tall you might want to reserve a seat on the overnight anyway to grab one of the extra leg-room ones.

If it was me, I'd want to try the London city flight back as it is a more unusual experience

The sleeper service is a good concept, but it only works if you fall asleep soon after boarding. If you are going to be awake for an hour or more after boarding then I would go for a non-sleeper and enjoy more of the CW service. Taking into account the jetlag, do you think you would be ready for sleep at about 10pm US time? If so go for the sleeper service, if not don't bother.

Have a great time.