I’m not sure about any air quality and humidity differences between the 777 fleets, I always feel about the same coming off any of them (dry nose and more thirsty than I would off any other aircraft type). I always find that it is a noticeably better experience coming off a 787 and to some extent the A380 too. And given a lot of the 787 & A380 routes I fly on are on the longer side when compared to the 777s it’s even more notable.

The A350 I find is a hit and miss experience when it comes to humidity and air quality.

Overall I end up seeking a 787-9/10 or A380 where I can for the F experience!