car rental in Madrid to visit north of Madrid.. Airport or in the city?

Closest to where you are when you need the car. Anything else is a waste of time and transportation expense. Plus, unless you're going up the road toward Burgos, it won't save you any driving time.

Hi Carlos!
We've rented cars in Spain 2 times and found driving in Madrid a challenge. The second time we rented a car we took a bus to Salamanca, spent the night and rented a car there - then drove north the next day. We dropped it off in Madrid on our return. Given the choice between the airport and the city, I think it would be best to rent and return it to the airport.