NW operates B747-200 (soon to be B747-400) from LAX-NRT and DC-10-30 from SFO. Personally, I'd pick the B747 -- especially if it's the -200, because it'll probably be your last chance to fly on one... both aircraft have plenty of "excellent" seats in Y, so that's not a concern.

FYI: I travel transpac in Y frequently... it's not at all bad.