Hi,
Thanks for the great advice everybody. It's all really helped.
I'm pretty certain that's it's China that I want to go to to study so I guess i'll have to consider Mandarin again.
Maybe i'll take the opportunity and try to get conversational to proficient in mandarin over a couple of years and I could always learn portuguese at a later date when I have more concrete plans to go to Brazil. I do really like the idea of having a life long hobby which mandarin learning could be.
I'm not a language expert; I'm just speaking as a native Spanish speaker and relating my own experience, so take it as face value:
I work with Brazilians on a daily basis, not only executives, who are (as a general rule) very familiar with Spanish, but often with blue collar warehouse workers who speak only Brazilian Portuguese. I've never had a problem understanding what they say or making myself understood. Some people have heavy local accents (i.e. Bahiano or Cearês) but nevertheless, if we speak slow enough and without slang we have no problems holding a conversation, even on the phone. (BTW I've never had a Portuguese lesson in my life but once you spot the very standard differences in grammar and phonetics, it all becomes very obvious and easy to grasp). I'm fluent in English so if we both speak conversational English we might switch to it for some short periods or isolated phrases but otherwise there is no problem and we both end up speaking Portuñol. I think that if your goal is to have a perfect Spanish or Portuguese there might be some interference, but otherwise, for conversational purposes, I'd think that speaking Spanish would be a huge step into learning Portuguese (or vice versa).
Having said that, I wouldn't miss the opportunity to learn Mandarin but that's a personal decision.
Mis dos centavos/ meus dois cêntimos.
OP,
I understand you're dealing with two issues:
Mandarin,
and
Spanish or Portuguese.
I'd try Mandarin. Even if you're not going to be fluent, not even close to, it will be a valuable experience. Learning a foreign language isn't only about a language. Through language you learn also about the culture and the way people think in that language. It's a unique opportunity for you.
Leave Spanish (or Portuguese) for later. From my experience, learning two languages that are closely related, from the start and simultaneously, is confusing. You tend to mix up words. It's much easier if you learn two unrelated languages, like Slavic and Romance or Slavic and Germanic. But, generally one language at a time is the most efficient way, if you want to become fluent some day.
What the last poster said wouldn't be very useful for you. He speaks as a fluent Spanish speaker that you're not and won't be in the nearest future.