You can't accrue AS mileage (either EQM or RDM) on HU, but you can on NW. The mileage from a SEA-PEK flight is worth a couple hundred dollars by itself, as well as taking you almost a quarter of the way to MVPG requalification (SEA-NRT-PEK is 6098 mi). HU doesn't have anywhere near the mileage redemption options that AS does (no US domestic airlines, and the overseas ones are ones like Air Berlin and Malev).

In addition, the NW A333 seats in coach that are available to AS elites for the SEA-NRT leg (rows 10-28) are pretty nice: 33 inch pitch, IFE, and power.