For me custard creams are the most aspirational of the two, unless the rich tea biscuits are the chocolate covered ones. Neither are posh really, you need some hand baked cookies/cream filled macaroons or similar for posh. Belgian chocolate butter cookies....mmmmm yummy.
Custard creams are definitely less posh. DH insists on having custard creams, bourbons and jammy dodgers in the house. If they're crawfords he's happiest. I call these his 'common biscuits'.
Me, being posher than dh obviously, bought cadburys chocolate digestives the other day and DH could not believe they were cadburys and not mcvities. He thought me the epitomy of posh. Its a good job I didn't buy green and blacks eh ?!