Well, exclusively focusing in the actual question of the thread
I would recommend the 5D mark II, bought it used in April and can't be more happy about it. It was such an improvement compared to my old 10D that I used for 7 seven years. I even asked myself why I did not buy before.
Reading about the 6D it seems that truly is a first FF for those who have had APS-C and want to upgrade.
The fact that it has a slot only for micro SD and not compact flash makes me dislike it.
Wireless and so on is a great add on, but imo not strictly necessary nowdays.
5D mark II is still a marvellous camera.
The only reason I would buy a 6D isthe possible (
) better ISO performance.
Choose right and enjoy the shooting!