I'll try the DG and compare it to what I currently use. It's on sale at our local CarQuest for $4.99.
Were the gallons on sale/in stock? I doubt seriously that you will be disappointed in the product. It is just a car wash soap, no miracle product, but it works and for me having access to gallons has been a real bonus as it is one of those products that I tend to run through with great frequency. IMO you would be well served to go ahead and spring for that gallon, if it is available.
Admittedly, I am prone to picking up car wash soaps (my preferred brands) whenever I find them on sale. Unlike some of the other products I seem to stock, the soaps/cleaners rarely gather much dust.