While they are made further along in the manufacturing process than acetates, test pressings are usually the first discs made from production stampers, so they will likely sound better than commercially available, or “stock” copies of the records sold in stores.

Do original pressings sound better?

Specifically, “Are the first versions of an album more valuable?” And, “Do first pressings sound better?” The answer to the first question is yes. First pressings of a vinyl record are generally more desirable. They're seen as more “authentic,” and so vinyl lovers will generally pay more for that first edition.

Are test presses valuable?

Test pressings are rare in themselves, although their value is very subjective. A serious Roberta Flack collector would definitely be interested in your one-sided demo disc.