Ironically your question has the solution to your inquiry in it. You are asking if you should try A or B. TubeBuddy has a feature in its subscription plan called A/B Testing which allows you to test different thumbnails on the same video to see which one gets better analytics plus it can also help test other things like different titles, tags and descriptions. Granted it's in the Legend tier, the most expensive tier, but it's worth it. But you can still accomplish this without TubeBuddy by manually changing the thumbnail at certain intervals and watching how your video stats change. TubeBuddy simply makes it easier to switch the thumbnails (and other sections) at preset intervals and collects the data for you which it then provides a comprehensive analysis of the different thumbnails.