I won't even thing in comparing an European capital such as Madrid with a South American Buenos Aires...first let's talk about the currency, you will pay in Euro not in the devalued peso...second let's talk about living in a cosmopolitan city and with all the respect and my love to south america and Buenos Aires but you won't see the snobism typical of south american countries still impregnated with colonialism...Spain is racist, but specially with la for all the immigration...besides,your experience may depend on having european passport or not or if you have chances to get a visa to study or partially work...
Madrid has a great night life, delicious food, great wines (not than here is bad, malbec is great too), beautiful people, pretty much as Buenos Aires, but don't forget that location and heritage defines pretty much the culture around.
It depends about what is your objective,what do you pretend to achieve and what are your expectations. If you have some money to spend, so why not to have a nice time in Madrid, but if you are looking for opportunities, check out your legal situation there before taking the chance.