Most riders do not pay any bribes in Argentina. Most of those who do pay bribes, needn't and shouldn't.
There are some famous roadblocks with corrupt police on the other side of the country from where you're going, between B.A. and Iguazu. None on ruta 40. None at border crossings. Don't give in to any hysteria on the subject. Even if you do find a corrupt cop, you'll find their efforts laughable compared to what's found in Africa.
The fact that you're asking which currency to bring to Argentina suggests you're not paying attention to the emerging black market. Bring US dollars and be prepared to change for pesos cautiously once inside the country. Lots of current information on the HUBB or ADVrider (or Lonely Planet, if they ever reconstitute themselves).
Hope that's helpful.