I've also flown both, if UA it's ORD/SFO/LAX - NRT, and if NH it's JFK/SFO - NRT (both directions for both airlines). I prefer the NH experience more than the UA one in two areas. One is that the food offerings on NH are more substantial than UA, and that between meal services a snack bar is set up in the in the Y mid-cabin galley for you to pick and choose drinks and little munchies (granted, they're Japanese but IMHO not too weird to a typical American palate). BTW on food, NH offers many of the medical special meals that UA discontinued a few years ago (I can use the low fat, or LFML which I can get on NH). The other is that from a sleep standpoint it just seems to me the cabin is quieter; it's as if all the Japanese go to sleep as a group when the lights are dimmed, hardly any reading lights are on, and there's no movement at all.

I won't comment on NH seats because I'm ethnic Asian on the smallish side and haven't found seat width or seat pitch to be issues. A suggestion is that you check the NH forum to see what folks have written on the ANA seats, as well as comments made for specific seats in Seatguru for both UA and NH. If it's UA, I prefer the PMUA 777 seats over the 747 seats. Somehow those 777 seats seem to fit the bumps and hollows of my back better.