Seville or Madrid?

Both are nice. I do think Seville is better given what you described.

Madrid has a lot of amazing art museums, that's one of the main draws. If that's not your thing, there is also great food. If you are into dance clubs/big city nightlife I think Madrid is also known for that. I'm more a wine bar/low key bar type of nightlife person, so I don't know for sure.

Seville has excellent restaurants and bars (tapas, wine, all sorts), it's the home of flamenco if you have interest in seeing a concert. The Alcazar palace is so interesting, and the gardens in it are beautiful. I personally find the atmosphere of Seville more charming than Madrid - lots of great streets and fun buildings. It's just a beautiful, fun place to be. I don't find Madrid as atmospheric.

Madrid is a city that never sleeps. Big tapas and bar scene, Retiro Park, the Palace and surrounding green space, Caixa Forum (modern artistic environment - small so not overwhelming). Modern food markets (prepped food stalls)
Seville can be a slower pace but you can find the tapas, bars and food markets there as well. Great river walks and head over the bridge to Triana. Plaza de Espana is great and the adjoining park has some small less visited museums housed in the original Ibero-American Exhibition of 1929. If interested, there is the bull fight or at least a visit of the bull ring.
Let's be realistic. Both cities have young couples that live there full time and Spain keeps late hours. You shouldn't be bored in either city.

If you are only going for two days, I'd go for Madrid due to better connections for US flights. While Madrid does have museums it is also home to a huge amount of young professionals like yourself. There's Retiro Park, the new pathway along the river as well as mercados, flea markets and more bars, restaurants and shopping.

If it were me I'd go to Madrid, there's good restaurants with food from all over Spain and other countries.

The only reason I'm voting for Madrid is the easier flights from the US. Have you considered Barcelona? You and your boyfriend could take a Game of Thrones tour of Girona.

I like both Madrid and Sevilla for different reasons. Madrid is hip: great food scene, art (not just paintings), and events, especially in May. Here is a link to all the festivals: https://www.madrid-traveller.com/things-to-do-in-madrid-may/. Also, if you are a football fan, Madrid is a great place to be if your trip aligns with a game. As others mentioned if you are flying in from the US, easier to get to and since you mentioned prices, the flights may be more attractively priced.

Sevilla is more laid back and for someone who loves GOT, the Alcazar is a must see. There is plenty to do in Sevilla as well. Some say it is the birthplace of Flamenco, the food and tapas scene is fun. Here is a list of the festivals in Sevilla: https://www.spanish-fiestas.com/seville/festivals/.

In terms of prices, I would look at booking.com before you go. I find prices similar for lodging, unless there is a big event going on that week and then prices can be higher.

Have a great trip. This will be a memorable 30th birthday celebration.

Sandy

I never assume that a poster is only from the US....but maybe GofT is the give away?
With further thought, what are your dates? There are some games and events that are going to drive up the price of accommodation in one or both cities. As to day to day spending, I didn't think there was much difference.
Feria in Seville early May http://www.andalucia.com/festival/seville-feria.htm

Oops sorry everyone, we live in London so flights to either easy enough and a similar price! :)

Oh - since you're in London - Seville is the winner!

Another vote for Sevilla. It has an atmosphere Madrid can’t match. Madrid has its own vibe, but it’s different than Sevilla. I like both cities. If you’ve never been to either, then you can’t lose.

Vote for Madrid! Because I'm living here! :D You will not regret

Considering you're just a short flight from Spain, then yes go to Seville and next trip to Madrid!