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"?