Excellent points, I.B.
It's better for a couple of reasons. First and foremost, you can most likely find a powder that works well for you that will cost much less than a liquid that works well. I have been using powder for almost 20 years and have periodically experimented with liquids, and in my experience the cost is roughly 50% less. The second reason is that you have more control over the strength. Liquids that are applied using metered injector tips can only be applied at the strengths that correspond to each tip size, but not in between. Too often, I have found that one tip size is good for the cost per car but not quite good enough for cleaning the really dirty cars. But the next tip size up, while cleaning well, results in an unacceptable cost per car.
With a powder, for example, I can strengthen up my mix at the end of a 3 or 4 day drizzly period because I know I'm going to see a lot of road film. You can't get this kind of fine tuning when your only choice is the next tip size up or down, and that takes you out of an acceptable cost range.
If you look at the ingredients of your liquid, a little research will get you to a similar powdered equivalent to experiment with. If you are worried about bad results while experimenting, be sure to start much stronger and then taper back as desired. It'll still be much less expensive than using a liquid.