Is this an int'l/int'l connection? If so, nothing to declare. If you are "entering" Argentina (i.e., immigration/customs) and then leaving later, then you have to declare it. It isn't in transit if you leave the airport with it. Declare it on the form. They rarely collect the forms but then you'll have declared it. If they x-ray your bag, then it will be good you have it on the form.