Madrid or Barcelona?

Most can tell you your best bet is based on your own interests. This is how you would choose. You should research both cities in tour guidebooks or on the internet youtube tourist videos and choose that way.
Barcelona attracts more tourists than Madrid. They are nothing alike.

Most (but certainly not all) folks will agree that aside from its three major art museums, Madrid is less interesting than Barcelona. However, Barcelona's top sights are swamped by tourists these days, so that might be a factor in your choice. But if you are fond of funky, oddball architecture, there is no debate: It has to be Barcelona.

How many nights will you be spending in the city you choose, and what time of year will you be there?

Both cities offer a great visit.

Madrid has the Prado, which is fantastic. I am not a Picasso fan, so that museum on Barcelona didn't interest me.

Madrid has the Royal Palace and Plaza Major. Also, nearby Toledo and Segovia are great. Toledo is the former capital of Spain and more interesting, since it is older. It has a great cathedral. Segovia is nice and worth a day trip.

Barcelona has the Gaudi sites, especially Sagrada Familia, which is special. Also, there are several other Gaudi sites. The Cathedral and Las Ramblas area is great. There are a couple of nice museums in Barcelona, one in particular about its history.

Monserrat is worth a day trip inland up in the mountains.

I would probably pick Barcelona, since when you come back to Spain, you could do Madrid as well as Seville (the best city to visit in Spain) as well as Cordoba and Granada.

Both cities are great. Each one has a lot to see and do and the metro makes it cheap and easy to get around. Obviously, Barcelona has a beach (several, in fact) so that may be a consideration.

However, if I was finding it hard to decide, I'd begin by doing a comparison of the top attraction. Then again, being a mean penny-pinching Yorkshireman, I'd probably compare the cost of flights and accommodation and then go to the one that offered me the cheapest overall deal.

Both are very much worth visiting. If you’re just staying in the city, then Barcelona. If taking any day trips, or if you like art museums, then Madrid.

Been to both - tough call to decide between them. Because of the Gaudi influence Barcelona is more unique. If you only have a short time then I would pick the one which gives you the most time in relationship to your overall travel schedule.

Do you have an especial interest in art (I don't, so my view isn't worth much, but I understand from others that Madrid is best for this)?

Otherwise, I think Barcelona has more variety of sights than Madrid, but Madrid has better (i.e. more varied), day-trip options. Both cities certainly have enough to keep most tourists occupied for at least a few days.

However it also depends where else, if anywhere, you are visiting in Spain. Madrid is more central - literally and also for connections. So if you're also visiting the south, for example, than Madrid is easier/faster.

Depending on when you're visiting, the weather might be preferable in one or the other.

It's like asking LA or NYC? Please get a few travel books or look on the internet and see what appeals to you. Many people like the NYC big town feel of Barcelona, very multicultural and it has a gorgeous view by the water.

I liked Madrid, simply because it seemed more Spanish and had the museums and culture that I craved. But I would never choose one or the other for someone else.

Or, as geovagriffith suggests, Sevilla.