I find the 6D to be an excellent landscape camera. I often take it on long hikes and overnight hiking/camping trips and it performs great. What don't you like about it for trekking?
(1) For landscapes, you are generally going for as much depth of field as you can without running into diffraction, so typically f/8-f/16. Generally, I'm using a wide focal length 24-35mm, I try to keep the foreground interest elements at or near the hyperfocal distance for my lens - which yields a very high DOF. I focus on the foreground element either using the center AF point and re-composing, or one of the non-center points that work very well in decent light. At these narrow apertures, even if you miss the focus a little it will not matter, as nearly everything will be in focus due to the DOF.
(2) I have a 7D and 6D and agree the noise of the 7D is pretty much non-existant at ISO 320 and below. However, the OP stated he wanted to use the camera for taking pictures of kids. I also have two kids at home, and find the low-light/high ISO capabilities of the 6D to be terrific. I take lots of indoor shots of the kids at ISO3200 that look great. I don't even consider shots like that with my 7D - the noise would be impossible to clean up in PP.
(3) Depends on the photographer. I'm pretty anal, so like my landscape shots to be noise-free.
(4) The 17-55 2.8 is a great lens!
(5) Agreed, APS-C lenses are much smaller and lighter. Full format will give you better image quality at the expense of size and weight of the gear. That said APS-C IQ is pretty damned good, more than good enough for most people, so its really a matter of personal preference.
Thanks for your comments. Now I have more doubts about buying the 6D ;-)
The things I didn't like from it fr trekking were those I mentioned in the list (extra lens weight, AF, lack of flash...), but given your comments on the AF, I may reconsider.
Actually my current camera (30D that is still doing nicely!) has a similar configuration for AF points, but often it misses focus (at f/2.8 ), if I try to use sensors other than the center one. That is why I am a bit reluctant about that AF configuration.
The reason think the 70D's AF is interesting for me, is mainly because I usually take self-portraits over landscapes, and usually I am off-center. I cannot focus and recompose, since it's hard to focus on my face while I am not in the frame. So being able to focus off center in live view would be great for me. Just set f/10-14, focus on me in live view through a mobile phone, and shoot. Not sure how the 6D would behave with that.
And yes, you're very right about the low light capabilities of the 6D. I'd love it for indoor portraits, although it is not the main use I do with the camera.
If only Canon told us when 7DMkII would come out, I'd possibly forget about both and go for it ... ;-)
My main issue is the AF, I generally cannot easily focus and recompose, since I ofen take self-portraitsl.