I think you answered your own question:
"I understand eVENT is waterproof, and Pertex is described as "highly water repellent". So what do you guys think?"
That's the critical difference - one is waterproof and one is merely resistant...and which you choose depends on the environment you're in. Most folks who use tarps often use a Pertex (or similar) bivy as it deals well with condensation, dew, etc. If you're using an exposed bivy, then something like eVENT is a better option. Since you mentioned using it as a shelter by itself, then eVENT seems like the best choice.