How do you know there's not a perceptible difference? I don't know myself, I'm just going by what John Carmack has said, and having watched the PS3 play-through video of a retail copy of Rage, it does look like the PS3 version has more pop-in going on.
Both versions are up on youtube and both have some noticeable pop-ins, but for me it's on both very minor distracting

When John Carmack said that Xbox full install > PS3 partial, that was at his Keynotes at Quakecon while they didn't know for sure how much they are allowed to install at the PS3. He said, that they still trying to get as much space as possible from Sony and only later, not so long ago it was revealed that they got their ok to install 8GB which is considerable bigger than most games do. Guess this helped a lot with pop-ins.
After watching both versions on youtube I would agree that both look pretty identical
