There are three or four wireless carriers here in Portugal, all with fast, low cost wireless broadband including Vodafone, Optimus and Portugal Telecom's TMN. Each have comparable pricing but after some four trips here in the past two years or so I've come to the conclusion that:
TMN is best for smartphone voice and data, while Optimus is greatest on tablets and inside data cards and MiFi's. Vodafone is the most convenient, with a shop at the airport.
Now where the differences are:
Vodafone caps you at 25 MB a day of data
Optimus is fabulous on tablets and for dongles/USB sticks data cards offering 1 GIga plans that are 10 Euros via PhoneHouse or direct.
TMN's 4GB plan at 28 Euros can't be beat on the iPhone, BB or Android smartphones
The best way to compare what's available remains going to the Phone House as their staff are on top of what's what, who has the best deals, and will even cut a SIM down for you, as will the Vodafone shop at Lisbon Airport.