Gringoed,
What is it that you want out of your trip? Is it strictly to lay as many girls as possible? Or do you have any other hobbies or reasons that you'd like to visit Brasil for other than just the girls?
Yes, Brasil has been getting very expensive (most expensive place in SA). Based a recent global study, the cost of living in Sao Paulo is as high if not higher than in NYC and Rio is not far behind that. So yes, you'd need to have a good amount of cash to really enjoy Brasil.
Argentina's main advantage IMHO is that the exchange rate is awesome now, the Ar. Peso is quite low compared to the US$. Yes, the country is very beautiful, the people are ok I guess, the steaks are out of this world and the wine is top class. To be honest, I wasn't impressed by the women in BA when I was there. Granted I was there only 1 week, but I didn't see many that made my head turn. When I was there, I couldn't wait till I returned to Brasil and as soon as I got in Foz de Iguaçu, in the Brasilian side, I immediately noticed a big change in the quality of women. Now, Foz is nothing to write home about as far as the quality of the girls are concerned, but it was obvious that women were sexier there than in Buenos Aires. My head was turning left and right to see with delight all these nice girls dressed sexily, the way that you'll rarely if ever see in BA/Argentina.
So what does Brasil have to offer? Everything you want and hope and some more.
You want beaches? You have some of the finest in the world. A couple of hours outside of Rio, you have the Paradisical beaches of Paraty and Ilha Grande. Further north, the beaches of Bahia, around Ilheus are top notch and not to mention those around Morro de Sao Paulo. The beaches of Ceara in North East Brasil are some of the nicest you'll see anywhere in the world. If you want to truly be blown away, go to Fernando de Noronha. Beachwise, I'm not even mentionning the beautiful beaches that line up the SP state and the insane ones in Santa Catarina in the South with Florianopolis taking the palm of the greatest and most relaxed beach town in all of the Americas. If you want a truly nice and sereine get away, go to the Ilha do Papaguaio, near Floripa. You'll be mesmerized.
You want culture and history? The huge centreal state of Minas Gerais is your destination. Coble stoned historical cities and tons of history will keep you busy for days there. Places not to miss in Minas are Ouro Preto, Mariana and Sabara and of course, the capital of the state, Belo Horizonte. The food in Minas is very hearty but delicious. Try the Tutu a Mineira or the Carne Secada. Delicia!!!!
Another must visit for culture is the state of Bahia, specially if you're black. Bahia is the place with the highest concentration of black people outside of Africa and of course, it's a centre of African-Brasilian culture. Very enriching, whether you're black or not to learn more about them and the way still practice their African ancestral religion and beliefs of Condomble, with their Orixas. Truly fascinating stuff.
You want eco-tourism? You'll have of the best options in the entire world: the Pantanal, the largest swamp in the world in the South West and up north, you have the Amazon rain forest, the biggest forest in the world. Each of these natural wonders can keep you busy for centuries.
What else could I tell you in a nutshell about a country the size of a continent, which is the 5th largest in the world in size?
You'll be truly blown away by the out of this world tropical and truly exotic fruit juices from the Amazon that don't have any English or any other language translation other than native Indians of the Amazon. The people are some of the warmest and most loving you'll meet anywhere. The scenery is breathe-taking. The futebol is insanely good.
On the flip side, yes it's expensive. But if you get to know locals, they'll show the places where you can save a ton of cash and no you won't have to stay in a favela. You can easily get nice appartment in a good safe environment for relatively cheap. But and this is a big but, Portuguese skills are required for that. If you or anyone truly interested in spending a good time in Brasil and you want to really get the most out of your trip, invest the time to learn as much Portuguese as you can. It'll make a world of difference to your experience. With little to no Portuguese, you'll just scrap the surface of what this mesmerizing country has to offer.
Also, be careful, it can be dangerous at times, specially in places like Rio and SP. Use common sense, don't flash money or any sign of wealth in public, so don't flash your Iphone or Ipod in public or god forbad, your laptop. The only places I felt confortable doing that was in Brasilia and Curitiba and during the day. In Rio, nightime, cabbing it is a must, at least to me. Don't take the bus in Rio unless and until you're either near fluent in Portuguese, know your way very well in the city and are accompanied by a local that you truly trust. The mini vans are safe. The metro is ubber safe, probably the safest, cheapest and fastest medium of transportation in Rio.
What else? Get to know some Brasilians online before you come down and have them show you around, both guys and girls. Guys are good to go clubbing with and to introduce you to nice and sexy girls. Girls are good too to make friends online before you come as you'd have a local guide and if she's hot, you can tap that ass after 2 or 3 dates, sometimes earlier depending on where in Brasil. But do learn as much Portuguese as you can. I can't emphasize this enough. The more you know by the time you're in Brasil, the better your time will be. Exponentially better.
What I usually do, I come to Rio for a few days where I party very hard with my friends, then I go to other cities to join other friends and that's where the real party begins.
As to making a comparaison between Argentina and Brasil, based on my experiences in both countries, Argentina is a beautiful country. BA is specially nice, relatively cheap and safe. However, I felt that Argentina would be more a place to spend a nice romantic weekend or week with a special girl you meet in Brasil. Girls from Argentina can't hold a candle to Brasileiras in terms of sexines, being sensual, feminine, positive, up-beat and all around so much more funner to be with. At least from my experience with both Argies and Brasileiras, to me it's 100% Brasileiras, that's not even a context. So get yourself a sexy and fun girl in Brasil and go with her spend a week or so in BA and you'll have a blast.
When people ask me if it's true that Brasil is a dangerous place, I always tell them that yes it's true. Brasil is very dangerous in the sense that once you get there, you'll be hooked for life.
Hope this helps. Mind you this is very general and broad, from the top of my head, at 2.30am. Any specific questions on the above, just ask!