Welcome to the forum and congrats on your new wash.
Powdered detergents can be fine. What used to be the big draw was the price. They essentially are shipping you the raw materials and you are adding the water on site, which is cheap and that way you are not paying to ship water.
Now they have ultra concentrated liquids, which again, are designed to have less water (more concentrated) these stretch further than conventional liquids- so you add it at your site when it is fed through your system.
The other thing with powders is that they often need some type of foaming agent and/or scent added to them. Simoniz Correct has a citrus scent already but most guys add a foaming agent to it. You will also have to mix your powders up in batches unless you have the strainer/screen set up for your equipment.
I think you can achieve good results with either ultra concentrates or powders. There are good chemicals in both varieties. As far as your salesman telling you that he sells liquid - it probably means that whatever line he distributes does not have a powder variety or he does not stock it. I would say more people probably use liquids, but you can be successful with either if it is set up right.
If you are brand new to the wash business, chems and equipment, it might be wise to let him set you up for now. Down the road as you learn more and get comfortable with things, you will be better suited to experiment if you choose.