We too, mostly stay at Hotel Europa at the Puerta del Sol. On our last trip we stayed at Petit Palace Posada Del Peine right at the Plaza Mayor, but only a two minutes walk from the Puerta del Sol. The included breakfast at the Petite Palace was good, but we didn’t care for the room. The Plaza Mayor/Puerta del Sol area is in the center of the historic district. If you planned on visiting many of the museums, then it maybe easier to stay in the Atocha/Salamanca area since they are close to the museums. Not knowing how you’re traveling from place to place, what might be nice if traveling by train is to have a hotel close to the train station. As a guide, Plaza Santa Ana is about a five minutes walk to either the Puerta del Sol or Plaza Mayor. From the Puerta del Sol, it’s about a 20 minutes walk to the Prado and Atocha. To me, most of the main tourist sites are walkable, but many might say otherwise. The metro is easy to use. All that being said, with only a week’s vacation, I’d stick to one country.