I don't know specific grammar points about "entä" and "entäs", but... In spoken language I would use "entäs". It would make the question more fluent and more easily spoken. I wouldn't use "entäs" when writing. I rarely need to use either of them. "Entä/entäs teille/sinulle?" is some thing that waiter could ask when taking your order. Where have you seen or used "entä/s"?