I have not flown in regular economy on United in a long time, but Economy Plus on United is by far the most spacious 'regular' coach product of any domestic U.S. carrier, and beats all the Asian carriers, too, across the Pacific.

As far as the seats go, NW easily has the most uncomfortable, poorly padded, and old/cheap looking economy seats of the majors. I flew them transpac a few years ago and they were quite awful. American and United both have new-style seats for the most part which are much more comfortable. NW is great if you're elite - I've always received an upgrade with them when I've had elite status with Continental - but in coach, it's quite dismal.