My advice is to pay with US$ at the ticket counter in the airport for the shuttle and worry about getting pesos after you get settled into your hotel. Pay the cab in US$ to get to the ATM from your hotel (I use the bank owned ones in the Palacio Municipale - I am sure I mangled that spelling) and get pesos. The scene in the airport is nuts enough without having to deal with it there, the shuttle tickets will be at or near the official exchange rate, and one cab ride paid in $US won't hurt that much.
Oh, and if you haven't seen this yet, do not get sidelined by people trying to "help" you as you come out of Customs; they are trying to steer you to their tables to snare you into timeshare presentations. Go straight to the ticket window, get your shuttle ticket, and go straight out to the loading area. They will start trying to talk to you about "free" rental vehicles, but nothing is free.