[My personal view as a beginner]
Both forms are acceptable and interchangeable in terms of politeness level.
The big difference lies on the speaker (circumstances, personality, mood ...). This distinction is often called degree of intimacy.
Intimacy - imaginary boundary (/fence) set by Speaker around him, which Listener(s) are not to cross.
읍/습 form. (deferential/formal) basically implies "Let's keep it strictly business" (or "Don't cross the boundary").
요 form (casual/intimate) implies "Boundary off", a way of inviting Listener(s) to further the dialogue.
In daily use, you will note that the 2 forms tend to intermingle in a conversation. That is because even during a friendly talk, small boundaries are set by the Speaker to fence off certain topics which he doesn't wish to dwell on...