You can find lots of answers to this question by googling. I found at least five pages about this in 30 seconds.
Most of these pages seem to say that if you must convert cash, converting it in the destination country is probably better, because you are essentially buying a product (in this case, dollars), and it will cheaper where the supply is greater. There are more dollars in the USA than there are in Portugal, so you may be able to get them cheaper there. (Some of those pages mention caveats if you're trying to exchange some little-known currency, which people might not accept, but this isn't an issue if you're converting euros.)
Some of those pages specifically recommend against airport currency exchanges; since they have a "captive audience" of people who want to convert money right away, they face less competition and may offer worse rates. Of course, the downside of doing the exchange in the USA is that you'll be less familiar with where to do it. I did find some people saying that, for this reason, it's better to do it in your own country where you can shop around at leisure to find the best rate. That said, if you take your time shopping around, shifts in the underlying exchange rate in the interim could erase any savings you find.
It's worth noting, though, that the main message from all these pages is the same: don't exchange cash at all if you can possibly avoid it. Use a credit card or ATM card to do the exchange. The exchange rate is usually better, and you also avoid the risks associated with carrying cash.