I had this same question just two months ago in June of 2017 . After doing some research I had decided to visit Barcelona and Madrid in Spain .

Now that my trips is almost over  I recommend both cities. Deciding where you go will depend purely on what you enjoy.

Barcelona

The good :

• Architecture- If you are a fan of Gaudi this is the place to be . I really enjoyed the Palace of Guel, Park Guel , Sargarda Familia and Casa Bastilla. All buildings by a famous architect with a brilliant imagination and a sharp sense of all the trades involved in building buildings. He was a genius and his buildings are imaginative and highly functional ! This would be my number one reason for visiting Barcelona.

• Food – Barcelona had many good places to eat at – Tapas, Pallas etc . You have to research and find the good places. The city is also filled with many many crappy places to eat at so make sure to research where you eat !

• Beach – the beach is very nice – the water was even warm enough to swim at night time . The beach is also filled with people even at night time so it is a great place to hang out at !

The bad :

• Crowd – this city is filled with tourists and the locals that are there to cater to tourists, because of this heavy use it is very crowded and very dirty, even when they clean every night ! Long lines for some attractions- typical of any famous place .
• Safety – there are a lot stragglers  in Barcelona that can make you uncomfortable as you walk through the quarters at night . Just always be aware of your surroundings and your possessions and avoid certain areas after dark !

Madrid

The good :

• Authentic – we visited Madrid after we went to Barcelona and this city is where I felt like I was in Spain! The places and people are more authentic! You get a sense for what the Spanish people’s day to day life is all about.

• Food – I found the food to be much better in Madrid! It even has the oldest restaurant in the world! The restaurant is called Botin !
• Shopping- Based on the places I been to in Europe ( Greece, Paris and Iceland) Madrid had the best value for shopping! I bought 17 tops !
• Safety – Madrid felt very safe compared to Barcelona. Never felt uncomfortable walking the streets late at night !

The Bad:

• Things to do – Madrid had a few things to do like parks, the Royal Palace etc but it’s not much and you can cover most of it in a day or two. Barcelona has much more variety!
• Weather – we were in Madrid in August and the weather was brutal ! Especially between 2 pm to 6 pm . Don’t go here in August!

I would recommend both cities – if you have the time ! If you can only visit one – pick Barcelona if you love Architecture and pick Madrid if you want a quieter vacation with more opportunities to experience the culture of Spain !