For architecture and landscape, the 24-70L II is the clear choice. For nature, I don't know, depends on what kind of nature you are talking about.
For portraits, it's almost a toss up. I don't think extreme critical sharpness is the most important thing for portraits, in fact just a little softness in the image can be preferable, not blurry, but not overly crisp, if you get my meaning. So in this respect, the improved sharpness of the 24-70L II isn't as important as the other features, which would be f/2.8 vs. IS & additional focal length when compared to the 24-105L. Frankly, for portraits on a cropped sensor camera, I'd say the 24-70L, but on a full frame the 24-105L might be just a little more useful, the extra mm on the long end can make a lot of difference for a nice portrait, particularly with the added benefit of IS. At 70mm, you are just a little short to get a nice head & shoulder shot, definitely for a close headshot. You get a little more out of focus background at 2.8, but at 4.0 it is reasonable for a nice portrait, especially zoomed out to 80-105mm. For bigger group shots at the wide end, the distortion is an issue with the 24-105L, the 24-70L II is probably better, but that would mainly be at 24mm I suspect, which isn't a good portrait focal length anyway.
For me, overall, I prefer the 24-105L for portrait work, but for more general purpose work, the 24-70L II is probably the better lens, over all.