I recently purchased the 6D and have since been thinking about changing the focusing screen.
I've toyed with the idea of the Eg-s but I also used an Ec-A focusing screen with the microprism in the centre on my 7D (purchased from http://www.focusingscreen.com
) because I use old manual lenses a fair bit.
I really love the modified Ec-A for my 7D, because it was so much easier to manual focus, but with slower lenses the microprism would go dark very quickly and this was definitely a pain at times.
I don't think there is much point in getting the Ec-A unless you manual focus with large apertures, but the website may offer something you would be interested in.
They are more expensive, as I think they're modified to fit the 6D, and they come from Taiwan (I think) so take a while to arrive but I loved mine in the 7D.
Sorry if I didn't really answer your queries but I think neuro summed it up pretty well.