EWR is easier to access from NYC. I will usually fly SEA->EWR and back due to the accessibility.

Since you are planning on staying at an airport hotel, that is a moot point.

EWR terminal for AS is pretty dated, only 2 options to eat (Ruby Tuesdays, Seafood joint) + deli style stuff. The lounge is pretty dated too.
I don't mind this though since I will arrive pretty close to boarding and almost always take the 9am or 6am flight back to SEA.