What causes more wear to the cassette --
Being in the wrong gear for the terrain (with a too-slow cadence, probably), or;
Shifting under tension?
Or maybe something else?
Yup. Ordered a rage red Rail 7 in November 2020, for March 21 arrival. That became May, then Sept, then Nov, then Dec, then Feb. To their credit, the shop didn't take a deposit, so I didn't have any money invested until it arrived.
Well, science tells us that orange and red bikes are the fastest. All other bikes are about the same. Seeing that your black bike will be just as slow (comparatively) as your spruce bike, just keep the black one. Should you have the option to get an orange or red one, of course, jump on that.
I don't think you can understate how much this weighs on buyers' minds. I've ordered a Trek from a bike shop 50 miles away that won't arrive until the end of Feb when I could have purchased the same bike from a shop 120 miles away. I just know my LBS wouldn't be thrilled about dealing with...