We like to have a week at each because they are such different areas.
You must be careful on the West End Road, especially when it's dark. But we have walked it many times. Alternatively, we take a taxi to whatever restaurant or pub we want to go to on the cliffs.
The cliffs are nice and quiet, which can be a reprieve after a week on the beach.
Having said that, we often get a taxi to the beach in the morning so we can have a long walk, returning to our cliff hotel for an afternoon around the pool.