Screen grabs with another 20 fps is certainly going to cause slight degradation of the image. This is an apples to oranges comparison and should not effect your basis for model choice. A better comparison would be full res still shots compared instead of screen grabs from different fps videos. The compelling reason for the black is higher fps at both 1080 and 720. If you are not into slo-mo then the silver with integrated touch display would seem to be a better choice.
GoPro Hero 4 Silver vs Black - Hypoxic Comparison
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