The downtown area itself is quite walkable though you'll need transportation to get to some of the suburbs. Buses require rechargeable magnetic cards (AR$45), which are available from nearly every kiosk in town. A single ride costs AR$15.38.
Taxis can be hailed anywhere; rides rarely go above AR$150, even if you're going to the suburbs. The green taxis (remises) are more comfortable; you're supposed to call those ahead but they will stop if you hail one in the street.
A car is very useful for visiting some of the nearby Jesuit estancias (ranches) that cannot be reached by bus. Depending on seasonal demand, economy cars cost around AR$950 per day with unlimited mileage. Try Europcar.