I guess I can't really agree with this. I like GH fine, and I like FR fine, but within the limited scope of Princes and Tomb, you're in the hills and temples in the former, and the jungle and dungeon in the latter. Within the context of whatever "fantasy world" it takes place, it doesn't matter because it doesn't have much impact on the story.
The only difference I would see is the deities a PC might worship, but that is still going to fall under the generic domains of Life, Death, War, Love, etc.
Now, if one were running a full fledged open campaign or homebrew, then the feel of the game world would have a much stronger impact.
Then again, I've never understood the Greyhawk/Realms love/hate thing. I like the Realms fine, but it's just a generic fantasy world for me with locations and some history that we have a consistent group memory with. If Greyhawk had been there more than the FR then I would have memories of that one, but either way, I don't care, so long as they have a fleshed out world to play in.