I've done both and would choose UA F if my main priority was sleeping. The seat is far superior to the spacebed seat.

The other thing to consider if you are going to be sleeping and travelling alone is the ability to book a window seat and be completely by yourself. I really liked this about seat 3J on the UA 777. I felt like I was in my own little world, which would not be the case in SQ C.

The IFE is indeed horrendous, but if you are just going to watch one movie and then fall asleep, it doesn't matter. If you plan to be awake the entire flight, then the spacebed is a much better option.

The United food has always been satisfactory. The SQ meals will be better, but airplane food is airplane food.

I was pleasantly surprised by the FA attitude on my most recent trips in UA F. They were cheerful enough and very attentive. Sadly, this is not uniform on UA. The SQ FAs are outstanding, period.

The new UA F is head and shoulders above the spacebed. But i think you're right that they won't be flying those birds from NRT any time soon.