Frankly I don't care so much for NH, especially in Y. Now if you're *G then you'll stand a decent chance of an opup on NH, but otherwise I find that the flight attendants are forever waking you up unnecessarily and serve the pre-landing meal far too early. I haven't been in the 777 seats in Y (thanks to opups), but the 763 on Asia routes is horrid.

UA has free beer (probably some other booze too) on transpac, so you'll be the same with that or DL.

As far as buying separate tickets, price it both ways but ex-Korea is often less expensive than ex-Japan.