Steam – What’s the effective difference between a Steam Wallet Code and a Steam Gift Card

If the game is an official Steamworks game there is no difference between the DVD version and the online version, the DVD is just like preloading the game. This means you get all the advantages and disadvantages of Steam, no matter where you buy it.

This is not necessarily the case for games that are just sold via Steam, but don't use any of the Steamworks features.

As Portal 2 is produced by Valve, the same people responsible for Steam, I'm pretty sure it will be a full Steamworks game and it won't matter where you buy it. I couldn't confirm it though, so I can't be 100% sure.

You can generally register all Valve games on Steam and add them to your library, so this will most likely also be the case with Portal 2.

The Big Picture Release Sale page on the steam website has an answer. Here's a screenshot of the right hand side from that URL:

And here's the text for completeness:

Full Controller Support

Games with Full Controller Support will work without a mouse and keyboard on hand, from installation to play and beyond.

Partial Controller Support

Games with Partial Controller Support may require a keyboard and mouse from time to time, during installation or for limited interactions.

So, basically, it would appear that a game with full support never needs input from anything that isn't a controller, whereas games with partial support can mostly be played with a controller but you'll probably need a keyboard and/or mouse to do something at some point (which might just be to click 'OK' on a window to install DirectX or enable controller in options on the first run or maybe click 'Play' in a launcher every run, etc).