B787 and A350 aircraft have the same C-class seats (unless you are on one of the ex-SU A350 - they have a different configuration - check a dedicated thread).
If you like to have good privacy, Instagramable seat but coffin-like small footrest - choose A350.
If you like to have great legroom, great wide footrest but little privacy (and sorry no Instagram pictures

) - choose B777.