Essential difference between the new full frame sensors is the pixel density. 18, 20, 22 mp.
Canon ff only have 21mp (5d2), 22mp (5d3) and 20mp (6d), the little difference is there to easily recognize which camera is "better" and more expensive :-)
Are you cocking a snoop at the 18 mp 1Dx ?
Agree with your analyses of the prosumer line. Difference is just for marketing, but maybe 6D users gained a benefit.