I think you will be pretty happy with either choice. I have a 5D3 and a 5D2, but not a 6D. I can tell you I am in no hurry to unload the 5d2. The 5d3 and 6d are newer, but don't forget the 5d2 is still an extremely capable body. The build quality is great, and if you are keeping the 40d, shooting with the 5d2 is very intuitve; it is basically a FF 40d. I went from a 50d to the 5d2, and thats what I found. Having said that, the 6d is newer, a bit more capable and a good choice too.