Spotless Franklins are by far the best bills to bring.
The rate differential from one cueva to the next is generally not critical unless you plan on exchanging an amount in excess of several thou, however, there is a cueva (in a travel agency as usual) in a mini-mall on Posadas in the rear of the Hotel Alvear that has been reliably correct in its rates.
You may get a slightly better rate if you exchange a large amount at one time, i.e., $1000 USD may get you like .10-.20 pesos more per USD than if you changed 100 USD. Until recently the ever increasing rate would have been reason not to do that, but the blue price has been dropping - that's like the sun rising in the west and setting in the east...but who knows how long that trend continues.