If you simply wash then wax with no other prep work, get the liquid, as Nu Finish is a cleaner wax (or polish as they call it, but the real purpose of their product is the same as a wax/sealant), the cleaning portion of a cleaner wax does a better job with the liquid version.
If you prep your paint with all the steps (wash, clay, polish), you could go with a paste to get the best durability out of the product. If you do it this way, you might as well get a quality long lasting wax (Collinite for example). Not everyone has the time or inclination to do all the steps, hence the availability of cleaner waxes (also called All In One).
Usually, you will do many more cars with a single paste can vs a single bottle, so the paste a better value.