As Port Town enters the battlefield, you may reveal a Plains or Island card from your hand. If you don't, Port Town enters the battlefield tapped.
: Add or .
4/8/2016 You may reveal any land card with either or both of the appropriate subtypes. It doesn’t have to be a basic land. For example, you could reveal Canopy Vista from the Battle for Zendikar set to satisfy the ability of Port Town.
4/8/2016 Lands don’t have a subtype just because they can produce mana of the corresponding color. Port Town itself is neither a Plains nor an Island, even though it produces white and blue mana, so you can’t reveal one to satisfy the ability of another.
4/8/2016 If a Plains or Island is entering the battlefield from your hand at the same time as Port Town, you may reveal the other land to have Port Town enter untapped.
4/8/2016 If an effect instructs you to put Port Town onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.
If you're playing aggro, chances are you want turn-1 mana fix and on this case the Shadow lands are better.
On the other hand, Shadow lands suck very bad for mid-range decks because you'll want at least 3-4 lands in play so top-decking one of these is really bad and even if you could play it, revealing the opponent you just have a basic land on your hand is bad.