SourceNeil Corbett from Rockstar, Lead QA on Grand Theft Auto IV, posted on the english forum rllmukforum.com discussing the differences between the two versions of the game and the Xbox 360 exclusive contents. This is what he had to say:
"Hello, as it stands the Xbox 360 version has the smoother framerate & far faster loading times (without any installing) but the PS3 version does have that warm feeling with bleached out sunsets & the like. The extra content of the 360 version is significant but nothing like an 'extra Island' at all- as the lead platform it is currently the superior version however. Just you wait until your first windscreen-flying experience."
So it seems he considers the 360 version having an edge over the PS3 one, even if he subsequently adds: "Suffice to say that both versions are stunning."