Here was a response I posted about these in a previous thread:
"I have all 4 of the threaded brushes, but I've only found use for 2 of them.
Black hard bristle - I used this to clean grout. Unfortunately I only did half our kitchen floor, so you can see which half was cleaned and which half wasn't.
Aqua brush - I use these to shampoo carpet and mats
White brush - I haven't used because it's slightly stiffer than the aqua and the aqua works well for carpet, so I haven't found a use for it yet.
Gray brush - It's a soft brush, but like the white I haven't found a use for it. I'm sure it would work well on leather, but I personally like wiping down and cleaning with a generic mf towel.
You also have to remember that because they're all threaded, each time you want to change a brush, you will need your wrench so it becomes a hassle switching from backing plate, to brush, to another brush, back to your backing plate."
Since then, I've used the soft, gray brushes to clean my leather. It was soft enough for that, but I found that to get it spinning fast enough to do any cleaning, it slung way too much product around.
The one you'd be using the most is the Aqua brush. If I had to buy it again, I would only buy that one brush and save the money on something else.
My $.02