Think about this in terms of your style. Both lenses, from my experience, are optically and physically great. Given that you mentioned "mainly walk-around" and landscapes, I think the logical choice here is the 24-105 f/4 L. For landscapes, you will most likely be using narrow apertures for the most depth of field in your images, so I think that f/4 will be adequate, along with the advantage of IS. For your walk-around shooting, the obvious winner, in my opinion, is the 24-105 L. You can't beat the versitality of the zoom range, and the IS will keep you shooting sans-flash in low light.



Since you mentioned candids in your main uses, here I would recommend the 24-70 L - I find it's bokeh worth it. But given you aleady own the 70-200 2.8 L IS (which is great for portraits), I don't think the 24-70 is that much more useful for portrait photography than the 24-105 L.



In the end, it all comes down to your own choice. I would suggest getting some time to use both lenses, even if it involves renting them for a little while. Shoot a variety of subjects with both lenses and get some real-world experience with the glass.



Good luck.