Citibank does give a much lower exchange rate than the regular bank rate. They admitted it when I asked them. When the dollar was trading at about 3.00 pesos per dollar, I was getting only 2.85 pesos from them. I stopped using them so I don't know what the difference is anymore.
Regarding getting a larger amount, I have noticed that the Banelco banks will only allow me to withdraw 500 pesos per day, whereas the Link banks permit withdrawals of up to 1000 pesos per day. But that may depend on the bank you use outside of Argentina and your agreement with them..