*thinks thinky thoughts*
I think my decision is based on shape and contrast. If the photo is about outlines of shapes - silhouettes or textures or un-busy geometric shapes, I'll run it through a bunch of different B&W treatments and see what I like best. If the photo is about contrast, same thing (all the better if the contrast didn't turn out so good in the color - like mountains).
If the photo is about richness or subtlety of color, it won't even occur to me to try it in B&W. Sometimes later I'll look at the photo through my B&W brain and go, oh! I was looking at that wonderful color, but look at the LINES! QUICK! TO THE B&W MOBILE!
I think going back and forth between B&W and color on many photos has taught me a huge amount about what makes a photo work....and what so definitely doesn't
Since most of mine are in category B, I must be learning lots, right?