In your image, you are basically trying to combine the foreground of the lighter exposure with the sky of the darker exposure. When you do this with a luminosity mask and the luminosity levels in the image you base the mask on have similar levels in the sky and the foreground, you end up making complex selections in the sky for example, instead of just selecting the entire patch of sky equally and making the mask pure white to composite the sky into the foreground. A pure white mask for the sky (versus one with varying levels of gray based on luminosity) lets the contrast in the sky of the darker image remain unchanged, instead of reducing it significantly as in your "cartoonish" image.
Instead of using a luminosity mask, use the blue channel of the lighter image as the basis for your mask. It has the best contrast between the sky area and the foreground. Using levels on the grayscale image that represents the blue channel, bring the the black and white points in toward the center of the histogram to force the sky to white and the foreground to black. You will observe that the histogram of the blue channel grayscale image has two humps - one on the dark end and one on the light end. These humps represent the foreground and sky areas respectively, more or less, so bring the black point to the right of the dark hump and the white point to the left of the light hump. See what happens?
Once the Levels operation is done, you will have a pretty good binary mask, but there will be light patches of pixels in the foreground area and vice versa - you can paint these areas out by hand. If you set the paint brush to OVERLAY mode, you can paint along the edge where the sky meets the foreground, with black for example, and the dark foreground of the mask will get darker, without darkening the light sky area of the mask. You can also refine this edge with the refine edge (mask, selection tool, whatever it is called now) and get the edge to appear natural when you composite the sky on the foreground image.
You can blend the darker sky image with the original sky to get the relative exposures of the two areas to look natural and then go about with your normal processing to get your final image. You can dodge/burn the rocks, for example, to create the effect of sunlight grazing across the tops of the rocks and the path, etc.
In this case, where you only have two images and the areas of the two images in the composite are large and well-defined, luminosity masks are unnecessary overkill and cause damage to the contrast if not used properly.