I stayed in San Telmo and found it perfect for Bs As. It has a good vibe about it, lots of good market area's, shops, bars and restaurants. Plenty good hostels (Tango was good). There are places where you can see a good Tango show and get a meal (Sunday night half price via hostels).
Easy to walk to Puerto Moderno which is pretty. Easy to get a bus to Boca to see the colour, beauty and of course football. Easy to get a tube to Palermo (good bars, restaurants, zoo, polo matches, expensive lol) or Recoletta. I'd say it was a good spot. Neither the cheapest nor the most expensive.
Bs As wasn't my fav place as not big into cities but it has a bit to see for sure.