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The difference in 24mm to 28mm is significant in my opinion, and I would not want to be with just a 28 as my widest option. For landscapes, any differences outside of pixel peeping or really big enlargements,or if the pictures were shot handheld in anything other than bright sunlight, would be marginal. I've had primes and zooms that overlap at 24 and 28, and once I get to f8, handheld without super high shutter speeds, sharpness differences for most part go away.
One thing I've also noticed is that some people confuse field curvature, with OOF areas, as soft. A good way to test is to use liveview and get something in the corner of the picture in focus, take a picture, and look at it. You'll be surprised as to how many lenses are adequate, or better, when things are in focus at the edges. If you think about it, pointing a wide angle lens horizontally in a landscape, chances are objects at the edges of your frame are much closer than the object in the middle of your frame.
Lastly, focal length selection is not just about angle of coverage. Perspective also changes with distance to the subject, and stepping back with a 28 to get a 24 angle of view will change the perspective of the photo; whether or not it matters is a different consideration.
I suggest getting a 24mm, and if you're not sure if you'll like that focal length, get the 24 f2.8AFD, shoot it stopped down, and you won't be out much $$$ if you decide you don't like it. And lastly, for a lot of people, 28mm is a perfectly fine wide angle focal length.
Hope this helps, enjoy your trip.