@Bassin' Brad, @Turkey sandwich and @A-Jay, I hope you don't mind my 2 cents here?
Because Colorado blades are rounder, they produce more vibration as mentioned and because of their shape, they do have more drag. However, you can drop down in size with a Colorado blade and get pretty much the same results, but you'd have to drop down even more than a willow blade to get the same effect. A #5 willow blade is like a #4 Colorado. If you dropped down to a #3 Colorado, you'd be at a #4 willow. The #3 Colorado will produce the same amount of drag and less flash than a #4 willow and sometimes that's just as desirable.
I do take flash into consideration at times. I do think there are days where too much flash turn off the bite. Even then, if you're not sure, you can get the best of both worlds with Indiana blades...but that's a whole other discussion...
Honestly though, I pretty much use a single willow blade on a snap swivel and switch out sizes, metallic and painted colors and you can pretty much run that that set-up for all your conditions. BTW, if you do use a snap swivel, you're gonna have to cut the blade arm of the spinnerbait and re-bend the arm for the snap swivel attachment.
One other thing that comes to mind. Another reason to "speed up", is when fish are nipping the rear of the bait or they come in "broad side" and nail the thing and don't come back. To me what this means is they saw something they didn't like and aborted the mission. By speeding up, they don't get an opportunity to see what their try to "blow up" long enough.
Sorry. Longer than I anticipated.