Which of these two are best to visit on a day trip? Thanks

by best I mean most interesting as to history, sights, views, etc

I'd choose Zahara.

But thinking about it, you could visit both in the same day.

Where will you be traveling from?

Olvera and Zahara de la Sierraoth are small white villages close to each other, unless your talking about Zahara de los Atunes, a small fishing village on the Costa de la Luz.

I understand it is Zahara de la Sierra, next to Grazalema. Anyway, there's nothing remarkable in Z. de los Atunes, but the wind.

zahara

though oif you have a whole day, start in olvera (castle for views, church) - you'll be done in an hour. then nip over to zahara...al lago is excellent for lunch on the terrace (al-lago.es), then the upper town, hike up the hill to the castle, back down to the main town for merienda and head over the top to puerta de las palomas towards grazalema in the evening for sunset views park walk etc. or you could sunbathe and swim by the lake in zahara all day. or go to grazalema for the day . or...