Here are my thoughts: The ‘Without’ version has still a bit too bright sky. With post processing it is possible to further darken the upper part of the sky. But we run into problems because the sky has been just too bright and cannot be edited well anymore. Often, when this is being attempted it will be visible near the horizon that the original file was just too bright. Using a graduated ND filter keeps me away from that challenge (even though it is possible to recover all details at times).
Which brings me to the ‘With’ version. The sky has more drama, structure, texture, details – whatever word you like. If you look at the horizon the sky is party yellow and blue. Also the colors in the clouds are more visible just because they are captured with a not too bright exposure.
What is not to dismiss is the possibility to use HDR technology, taking a bright & dark photo and combine them accordingly. Nothing wrong with that. Using the filters just allows you to produce one photo of the scene with the possibility of relatively little editing work.
Here is the setup I used for these photos:
Camera: Nikon D5
Lens: Nikon 24-120mm f4
Filter holder: Haida 100-PRO Insert Filter Holder
Filter adapter: Haida 100-PRO Adapter Ring,77mm (to fit the holder onto the lens)
Filter: Haida Red-Diamond Soft Grad ND0.9 Filter,100*150mm
Exposure time: 1/25s
Focal length: 24mm
More examples using the same equipment setup: