Make your decision based upon what YOU are interested in. The post immediately above mine gives you a lot of information that should help you decide.

If you're interested in art, e.g., there are at least three fabulous museums to see in Madrid, then you wouldn't want to miss it. And the great day trips that help you learn all about how the new world was "discovered" and colonized (to be where Isabel and Ferdinand made so many decisions that had such an impact on the world is awe inspiring - for me anyway).

OTOH, Buenos Aires is a fascinating place to visit because of the unique blending of cultures (Spanish, Italian and to a lesser extent German), the pampas, the food (GREAT beef, GREAT pizza, GREAT gelato, dulce de leche). The people are fascinating. I think they have the highest percentage of psychiatrists in the world. I like to think of it as the last remnants of the great society that never was - IMHO, because of the roles the Perons, the politics and the military played in screwing things up, combined with attitudes like the French.

Both are fascinating places. Madrid will be cheaper to get to. BA will be cheaper once you get there. Either way, you won't go wrong. If you're only going to be able to make one trip, ever, pick the one that sounds the most interesting to you.