Both cities are incredible and rich with culture and nightlife. I find the nightlife in BCN more interesting and fun but a little younger...Madrid's gay nightlife is in the Chueca which is nice that it's in one area but the clubs/bars can get a bit old pretty quickly. I find the architecture in BCN (primarily Gaudi) amazing and allows you to spend days just wondering the city.

As far as language...BCN speaks 3 languages, Spanish (what Americans would be familiar with and what they speak in MAD), Catalan and Basque...Almost everyone speaks English in BCN...In Madrid I find myself speaking more Spanish to converse with people.

Mardid to me seems more authentic Spain because it's seems more rooted in the Spanish history whereas BCN is more urban and has many more influences from outside of Spain.

You can't go wrong with either city...both are amazing and you will have an amazing time in both! Enjoy!