I can only speak for the 16-35 f4 as I've owned that one.
I'd say it's image quality wise it's very very good for landscapes, and if you're fine with it being f4 at its widest, an ok daytime walkaround lens when traveling.
It has those awesome sun-stars stopped down a little, is sharp wide open, great color and contrast, and I find that 16mm is as wide as I'd typically want to ever go.
I didn't keep it however because I tend to do a lot of lower light photography when I'm traveling, and I'm not much of a landscape shooter (I'm more about taking pics of people, not things) and I found f4 limiting. If you tend to do a lot of landscapes, I'd say it should be high on your list, if not you may found that it sits in your camera bag most of the time like me (and it's not a cheap piece of glass so I can really only afford having glass that I use a lot).
If you're only an occasional landscape shooter, the 28mm may actually be enough for you. Just something to think about.
I have tried out the 24-70mm f4, and for what it's worth, wasn't very impressed with it optically. It does focus significantly faster than the kit 28-70 though.
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