On the note of the two GTAIV versions being visually identical, I found this article today that seems to conclude that Take Two said the PS3 would be visually better, if only slightly.
http://www.palgn.com.au/article.php?id=10709&sid=344835db4f4f0e123d70c5b768291c79&title=Rockstar+sequel+coming+in+2009
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