seat size is essentially the same, very little in it as to which is 'better', both cabins feel roomier than the a330
air quality and noise levels are better in the 787
the a340 cabin has 1-1-1 layout, assuming you go for a window seat, whether you're facing the front or rear of the opposite seat depends whether you're in the front or rear of the UC cabin - i like 2K as it faces the rear of the other seat and there's no one in front (some avoid it as it's closest to the front toilet but i've never found that a problem)
the a340 cabins are older and perhaps more likely to have problems, last time i flew it the armrests etc. had faulty catches
a340 seat power needs an empower adaptor rather than standard ac plug, this year i think usb is being added but not sure when the rollout is complete, ditto for wifi
i hardly ever use the IFE except for the moving map so not sure if there's any big difference, probably the 787 has more bells and whistles and the IFE is less likely to be tired/faulty