Agreed - long flight and luggage, take a taxi. Esp if you don't know exactly where you're going. Do NOT try to figure out the bus with luggage, tired and not knowing BsAs yet.
And take a taxi from Tazi Ezeia. They have the airport concession, they are honest and trustworthy and will walk you out to the taxi and give you a receipt. Can't recommend them enough. You will see their stand as you pass after you clear customs, go through the little room (ignore the stands there) and exit through the sliding doors. They do take credit cards as well as dollars. No, their exchange rate isn't great so if you pay in dollars, they will charge you at the official rate (maso 45 USD) but again, they're legit and it's easy. Even better if you have pesos.
Once you get to your apt or hotel, drop off your bags and then start to figure out the bus and subway system.
And to the person who posted they paid 90 pesos for a taxi from the airport to town - um.. don't think so. I paid 52 yesterday for a taxi from Corrientes/Larrea to Libertador/Maure. 90 pesos is not anywhere within possibility for a metered rate. Maybe you worked out a special deal - kudos to you - but that isn't really helpful/realistic for a newcomer.