No experience with mk i, but I love the mk ii, the way it feels in my hand and how it performs, perhaps the buffer is a tad slow with burst but that's not a deal breaker for me.
Today I played with a 5D mk ii and I must say the 6D seems much superior, the way it felt in my hand, overall shots were much better on AUTO on the 6D compared to 5D, I guess you could tweak the images in photoshop but ain't no body got time for that.
I think the 6Dmk ii could be slightly better and perhaps I would be a bit more picky about it if I was a professional photographer, but if I was one then I would just get 5D mk iv or 1D.