If you buy the product via Gametap then you get each episode two weeks in advance of those who buy the game from telltalegames directly. Now this wouldn't be a problem if gametap was available all around the globe, which isn't the case. Actually it's only available in usa and canada.
Unless I'm confusing GameTap with something else I wouldn't use it even if it was available. GameTap, IIRC, is one of those rental sites where you only get to play the games while you're subscribed. As soon as you stop paying, the games are gone. But as I say, maybe I'm thinking of something else.