Developer Valve calls the PlayStation 3 version of Portal 2 "the best console version of the game", for various reasons. For one, buying that version gets you a copy of the game for Steam too. But more important, to Valve at least, is that it brings the Steam platform to Sony's console.
Valve marketing man Doug Lombardi explains just how Steam integration, saving to the cloud and getting your free copy of the PC/Mac version works. Steam on the PlayStation 3 means "cross-platform matchmaking, gameplay, friends, chat and achievements," Valve says, with PSN trophies and Steam achievements "earned in lockstep".
This may be the most important part of the deal, for anyone considering getting the PS3 version with the intention of playing it on PC/Mac.
Steam on the PS3 provides access to all of these features once the player has linked their PlayStation Network account with a Steam account, which is done right within the game. A Steam user can login to an existing account, and anyone new to Steam can create an account with the click of a button.
So, there you go.
Valve Reveals Portal 2 for PlayStation 3: Steam Details [PlayStation.blog]