richiexdee said:
I want to take photos in low light situations, landscape, sport... everthing with one camera
Welcome to Canon-land, this basically means you have to have two cameras (like a 70d crop + 6d ff) or the 5d3, that is if your lenses are long enough - and yours are as you've got 400mm with the extender.
richiexdee said:
Would the 5D III be an better allrounder as the 6D? The only things that will be better in the 5D III would be AF, dual SD slot and anti aliasing... I don't know if this would worth the 1000 € extra..
The 5d3 blows the 6d out of the water in features and usability, but only you can tell what $1000 means to you. The 6d basically is a good sensor in a bare-bones shell with everything removed Canon could get away with.
Looking at your lenses, a better af system would be useful because the 6d struggles with fast lenses (no cross point at all for f2.8, even the center is only f5.6 precision) and focus/recompose with fewer af points means way less keeper with a thin depth of field. And if you want to shoot sports as in "tracking", the 6d is simply out of the question, trust me, I've tried as a lot of others and the af system simply doesn't cope.