I have flown both UA and NH in First, and this is my observation:

Food: Simply no comparison at all. ANA would feel severely insulted to be mentioned in the same sentence as UA. Especially if you like Japanese food... you just have to try ANA.

Service: Again simply no comparison at all. ANA flight attendants really recognize that you're a premium passenger and act accordingly. They know how to serve the food and drinks in the right way. And they will make provisions for almost any request that you have. On UA, I had very plain service that was hardly differentiated from that of Y class.

Seat: This is possibly one area in which ANA is beaten. Admittedly, I enjoyed UA's seat much more than NH's, but that's only because I needed to forget the bad food and service on UA and wanted to sleep. On ANA I was too busy eating and watching movies on their Video Walkman... which brings me to

Entertainment: both ANA and UA have in-seat programming and video cassettes for personal video. I imagine there will be a similar variety of movies in both airlines.

Lounge: both airlines' lounge in JFK are very inadequate, I heard. In NRT, UA does have the best lounge in the whole airline and by virtue of NH being in a different terminal from UA, you will miss out on this particular anomaly of UA "service". In SIN you can use any Star airline's F class lounge so no difference there.

In all, the only advantages of UA are the seat and NRT lounge. ANA has an uncomparable level of food and service that makes you not want to leave the plane.