Not sure whether you'll get the PlayStation 3 or Xbox 360 game Portal 2 the day it came out? Know that Valve has the answer to your question:''The two versions are identical.''
Chet Faliszek Valve is reassuring:''If you buy the PS3 and Xbox 360 versions and put them next to each other, you will see that both games are identical. The content is also the same on all platforms.''
''We can perform updates more regularly with the PS3, we do not need someone to do it. This is one reason why it is easier to play games between the PS3 version and PC version (cross platform), the updates are much more frequent. We also added chat across platforms (PS3, PC) and the Steam Cloud for the PS3.''
By''cons, I recall that the PS3 and Xbox 360 versions of Portal 2 are identical.''
Remember that PS3 gamers are serviced Steamworks Valve allowing among other things, play with PC owners (cross-platform), which is impossible with the Xbox 360.