Ronda from Seville or Malaga?
 

As we will be in both locations, which would be better for a day trip to Ronda? I have read that the guided viator type tours that leave from Malaga end up spending far too much time in transit, picking up folks from many different hotels...

Why not rent a car and visit some of the Pueblos Blancos, next to Ronda?

Depending on how you're traveling (driving), if you're going from Sevilla to Malaga, why not stop in Ronda for the day? You can also take the bus.

Or you could take the MD train from Malaga to Ronda and spend one night. The trip is about 2 hours in each direction on the train. With its schedule, you would only have about 4 hours to see Ronda unless you only wanted to stay one night.

I don't want to stay over in Ronda, and we have Granada in between Seville and Malaga, so can't really do Ronda on the way. I think Malaga is going to be closer, perhaps we will just rent a car for the day....

From Sevilla or from Malaga, it won't make much difference. It's about a two hr drive from either location.
Check distances/driving times on www.viamichelin.es

From Málaga, you have a choice of fast motorway/toll expressway to San Pedro de Alcántara and all-purpose road to Ronda (about 2 hours in all), or a scenic but slow drive inland (3+ hours; can be monotonous at times). From Seville, the only route is via Villamartín, which is quite scenic for the latter part (about 2 hours in total).