I am also an HSBC customer and employee. If you are a Premier customer, then the previous posting is correct: you will not pay any ATM fees to withdraw cash from any HSBC ATM (we have a lot of branches in greater BsAs and in Argentina in general). However, you WILL be charged an FX fee regardless...this fee is quite nominal (i don't notice it personally, even though I get charged myself if I use my Premier account from abroad here). If you already hold premier status, then you will automatically qualify for Premier here in Argentina. In this case, I would recommend that you open an account (go to Casa Central at Florida and Peron if you need an English speaker) as you get free, unlimited transfers between same-name accounts. If you're not a Premier customer, then you can probably qualify here in Argentina as the amount needed in a deposit to qualify are significantly lower than in other countries.
As far as Citi goes, I have a friend who works at Morgan Stanley and has his accounts at Citi. He's been here in BsAs for 7 weeks now and still hasn't been able to receive a local bank account. I know that Citi is downsizing this year, so if you're looking for convenience, unless they are close to where you live or work, it might be more difficult. I have no idea what their fee structure is, though, so it might be best to check it out on your own beforehand.