Manaus is interesting - it's an island in the midst of the forest. But lots of poverty, lots of rain in the wrong season, and nobody speaks english. Most people going there for tourist purposes book a couple of days in a rainforest lodge. Buenos Aires is something totally different. 24hs a day alive, lots of culturally interested people, lots of bookstores, totally devoted to tango. Easy and cheap to rent an appartment in San Telmo or Ricoleta, good food everywhere, superb wines. Spend some time in a milonga (a tango danceteria or salon) and you'll be hooked forever. Meet one of those beautiful porteño women and you'll find it difficult to go back home. I've been there 5 times thus far, and would go twice a year if I could. Cheers.