I recently did SEA-NRT in E+. The 777 on the way down wasn't bad. The 747 on the way back wasn't nearly as comfortable. For some reason I find the seats on the 747 to make my butt sore. PTV on the 777, overhead TVs on the 747.
They were looking for VDBs both directions which included an upgrade to C for the next flight out (24hrs later). Perhaps if you have some flexibility in your schedule, this may help with the comfort level.
I slept a good chunk of the flight both ways.