Steam functionality is the key reason
PlayStation 3 owners who buy Portal 2 will be in for a big treat, as the Steam functionality found in the title will allow for a number of key cross platform abilities that are new. The cross platform abilities of the title will allow for multiplayer play against PC and Mac users, as well cross platform chat.
The Steam features that are being added to the PS3 version of the title are the key reason why this is possible. PS3 Portal 2 owners will also be able to link their PSN accounts with their Steam accounts. Xbox 360 owners are out of luck, as they will get Portal 2 but none of the Steam cross platform functionality. (The game will also be released for the PC and Mac.)
As an added bonus, PS3 owners who buy Portal 2 will be able to unlock a full copy of the PC/Mac version of Portal 2 once they link their PSN and Steam accounts. It is a nice bonus for PS3 owners.
It is difficult to say how the addition of all of this Steam functionality will play out on the PS3 platform, but it does offer cross platform abilities that could make it very interesting for PS3 owners to check out when it arrives in April.