I always say español. For me it's all about convenience, I'll explain my very subjective opinion. Doesn't it make sense to call the most representative language of a country after the name of said country? We do it with other languages (english, italian, french, german, etc...) and no one is saying that's the only language. Italian isn't the only language in Italy, there's other native languages as well. Same situation with other countries like Russia, Finland, France, etc.

When I was learning english, I tried language partners and got in contact with an english learner from Indonesia. I didn't know anything about her country (did you know they used to be a dutch colony?!) and she didn't know about Peru. When I told her my native language was spanish, she immediately made the connection to Spain. Castellano/castillian would've meant nothing to her. This is a minor detail but the fact that she was able to make that connection about the history of my country meant a lot to me.