I wanted FF. I really like shooting people and natural light is my favourite (and easiest!). My requirements were low noise at high Iso for natural light portraits.
This picture highlights a shot that would have not been as possible on my 600D:
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Paris by
acearchie, on Flickr
It was taken at ISO 4000 and through further experimentation I know that I can push it to easily ISO 6400 (as in the image below) and still really not have to worry about noise.
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Alex by
acearchie, on Flickr
In fact, I love the fact that my 6D can pretty much see better than my own eyes. The scene below was darker in real life yet at ISO 12,800 there is still low noise and it is definitely acceptable for a web use shot. This was the first shot I took from my camera after I took it out the box!
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Home At Night by
acearchie, on Flickr
Linked with this is the AF on the 6D. My 600D would search quite a bit but my 6D is great at hitting the mark (by default I stick it on centre focusing and recompose if need be). The centre point is the best at focusing in low light compared to the rest of Canon's range:
This is something that a lot of people gloss over when comparing focusing types. To be honest, I have shot with a 1Div and found that I did not need all the focusing points it had to offer. In my opinion, 9 is the sweet spot and I only use the outers for portrait orientated photos when there is enough light.
The other main reason for me was I wanted a shallower depth of field. I like being able to separate the subject from the background and depth of field is the quick way to do this. Having now got lenses from 11mm all the way up to 85mm I am able to use my lens selection to aid how much depth of field and perspective distortion I want.
The DOF is the main reason that I have been shooting MF film for quite a while. Whilst the lenses I have are slower the DOF adds a really different look and helps pop the subjects out from the scene.
The 70D wasn't out when I bought my 6D but I didn't even consider the 60D as it is the same sensor as the 600D and in essence apart from a few buttons changing around (which in itself can be quite important) it was practically the same camera.
It wouldn't have allowed me to take any different pictures from the 600D as the focus points weren't my requirement and on the 70D I view the video AF as gimmicky as in a professional video environment you always focus manually and I have therefore invested in kit to aid me with this.
Feel free to ask anything else!