I assume these are 100% crops of the full image.
How did you scan the negative? Is this film grain or scanning artifacts?
The difference is more evident if you convert the images to monochrome.
It is possible that Smart Convert and Grain2pixel use different demosaicing algorithms.
Personally, I would prefer a negative conversion software to apply a bare minimum of image processing. White balance, exposure compensation, contrast and sharpening should be left to the photographer.