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The title pretty much states my question here. I've held a T6i (very smooth feeling, good autofocus) and a 5D2 (definitely higher build quality, a full-frame which I would prefer). Never held a 1Ds3 but I did get an opportunity to play with a 1D2 and it felt old and clunky, although well built.
I'm not too concerned about video quality, or even whether video is possible - I don't shoot video seriously whatsoever. I do enjoy good resolution (20MP or above seems to be comfortable) and subject tracking/AF during continuous fire. I'm also definitely going for a Canon since I can adapt my dad's professional M42 lenses - no AE/AF but buying new lenses that cover the same breadth of focal lengths would cost thousands.
I was leaning towards the 5D2 for it's full-frame sensor and high ISO capability and current price point. However, a recent trip to a camera shop where I got to play with a T6i and a 1D2 made me reconsider - the T6i focused much faster, and the 1D2 definitely felt like I would damage the floor more than the camera if dropped. I don't have a particular area of photography I'm interested in, but I'd like the camera to last and take high quality images. What's a good choice here?