Madrid or Barcelona

How do you feel about art museums? If you're not much of a fan, I'd throw the extra day to Barcelona, where many of the top sights require prebooked, timed entry tickets. That makes it hard to pack a lot of sightseeing into a single day. Big-time art lovers might spend about two full days in Madrid's art museums, so that could change things. Madrid isn't as old a city as Barcelona and it doesn't have modernista architecture to admire (or laugh at, depending on your point of view). To me, Madrid just isn't as distinctive as Barcelona, visually.

I am not suggesting Barcelona has no art museums; I've been to many and liked them.

There are more sites in Barcelona to visit and four days would just be scraping the edge of what's needed if you've never been to the city before, although las Ramblas is simply not the "must see" thing that so many guidebooks/articles claim it to be.
Madrid has more museums, but there's not really an equivalent say la Sagrada Familia or Parc Guell in Madrid. You can enjoy yourself very nicely in Madrid and have a chill time for a good deal less money however.
You also didn't mention what time of year you'll be visiting. The tourist crush in Barcelona is absolutely horrible from later June to early September now so if that's an issue, Madrid will be somewhat nicer as visitors are more spread across the city.
For either city however, book tickets ahead of time and be well planned. Like most of Europe at this point, casual drop in visits aren't a thing anymore.

I like both cities, but Madrid has nearby Toledo and Segovia that significantly enhances a visit to that city.
Toledo was the capital of Spain 500 years ago prior to it moving to Madrid. The city is well preserved, and very, very special. My favorite city in Spain.
Segovia has an ancient Roman aqueduct that still functioned until the mid 20th Century.

Not being a museum person, I’d give the four days to Barcelona. If you intend to take a day trip to Toledo or Segovia, then Madrid would be my choice. Both cities are large, busy, and have plenty to see.

@geovagriffith... Toledo was the capital of the Kingdom of Castille. Spain as such did not exist until the military conquest of the territories of the Crown of Aragon (Aragon, Catalonia, Balearic Islands and Valencia) by the Kingdom of Castille, and the dismantlement of their armies and governing bodies... which happened in 1716 (Decreto de Nueva Planta).

I also agree nearby Toledo and Segovia enhance the visit to Madrid. But equally, nearby Girona and Tarragona -as well as Montserrat- greatly enhance any visit to Barcelona, and then, of course, Costa Brava ;))

I have been to Toledo twice and both times our guide told us Toledo was the Capital of Spain.
https://www.travelguide-en.org/toledo-the-historical-capital-of-spain/

Yep, I’m sure you've been told so—very imperialistic indeed—but basic History 101 shows otherwise.

This is a common misunderstanding, analyzing something from the Middle Ages with a 21st-century perspective when in reality, it was entirely different from what we would interpret today. During the Middle Ages, many royal "marriages" in Europe did not merge territories and possessions into a larger entity (a supra-entity), but were instead purely dynastic unions. Each sovereign—or each dynasty—kept their domains intact, and legally, one ruler did not govern the possessions of the other, nor did they become part of their inheritance. These dynastic unions functioned as alliances of convenience—what we might now call joint ventures—for as long as the marriage lasted. They conducted joint operations, such as financing military campaigns to expand their possessions or establishing commercial alliances.

However, each territory retained its own currency, laws, armies, and institutions. Thus, although these unions sometimes operated de facto as a single entity, they remained legally separate, and in some cases, they dissolved after the death of one of the spouses. A clear example is Isabella and Ferdinand (the Catholic Monarchs). They ruled separately as Queen of Castile and King of Aragon. In fact, when Isabella died in 1504, Ferdinand ceased to be King (Consort) of Castile, and the crown passed to their daughter Joanna (Juana la Loca).

Before 1716, Spain was not a unified state, but a composite monarchy, where different territories had separate laws, courts, and government institutions and there were even internal administrative borders -terms that define self-government in classical politics. Even under the Habsburgs (1516-1700), Spain lacked a centralized government.

However, this changed after the War of Spanish Succession (1701-1714), often considered the first de facto "world war" in Europe, as it involved all the major powers. Philip V, alongside its French counterpart, militarily conquered the Crown of Aragon --the confederation of Aragon, Catalonia, Valencia, and Mallorca-- which was backed by Great Britain, Portugal, Prussia, the Holy Roman Empire and the Dutch Republic, and centralized power with the Decrees of Nueva Planta (1716). He abolished government institutions of the territories of this confederation and disbanded its armies, imposing the Castilian system. This marked the creation of Spain as such.

Similar European examples of such dynastic unions:

• Poland-Lithuania (1385-1569) – A dynastic union through marriage, which only became a formal state with the Union of Lublin (1569).
  
• Britain-Hanover (1714-1837) – British kings also ruled Hanover, but when Queen Victoria inherited the UK, Salic Law split the union.
• Austria-Hungary (1867-1918) – A dual monarchy under the Habsburgs, which collapsed after World War I and the abdication of Charles I

... sorry, I am a bit of a History buff :))

Enric,
You many great points with your historical background. I think the difference in opinion is whether you accept the Castilian or Catalonian view.

The date want dicussed? If iits June, July or August I would pick Madrid and the nearby Toledo and Segovia.