1 day for either Salamanca vs. Toledo

I think Toledo has more sights and is more dramatic, though Salamanca is certainly a worthy destination.

What I don't know anything about is dealing with a car in either city. It's possible Toledo would be trickier. The historic area is up on a hill. If you have to park below, possibly some distance from the elevators that assist with the trip up (there are also buses), that would take some time out of the less-than-a-day you have available. Toledo, by rights, needs more time than Salamanca.

Where are you driving from? Salamanca is in the far west of Span near Portugal and Toledo is just south of Madrid?

Both a wonderful, but I would go with Toledo.

Going to Sevilla, Toledo is closer if you take the A4. You could stop at Consuegra (a couple miles west of the highway) to see its castle and windmills. Also off the same road, but east of the highway is Campo de Criptana and its windmills referenced in Cervantes Don Quixote. If you go to Sevilla via Extremadura, then Salamanca might make more sense. If you’ve never been to either city Toledo would be my choice.

Regarding parking in Toledo, there is a pay parking garage in the historic center just past the Alcázar. Enter the old town by the Bisagra gate and follow the road. Just past the Alcázar and Zocodover Plaza the road makes a sharp left and the garage is on the right. For free parking there are a couple lots by Avenida de la Cava near the escalator (Escalera mecánica del Parque Recaredo) that goes up to the historic center. One is at P.º de la Basílica. On Google maps just enter “Aparcamiento Gratuito Toledo” The Puerta del Cambrón is across the street if you don’t mind walking up the hill.

Which wine region are you driving from?
If it's the Duero, then Salamanca is close and makes a ton of sense.
If it's the Rioja, it is also best to drive via Salamanca rather than via Madrid, so Salamanca is best there too - but it is a long drive so you won't have much time!