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I think the question was "What are the differences?" Not "Are there differences?" F.L.Y. ;)
And to answer the OP's question. I have no clue. I know there are differences, but I don't know what they are yet. If you find out though, be sure to post so we know. :)
An interesting side note. The Main Pokemon on Pearl is sort of the Patron God of Space, while the one on Diamond is the Patron God of Time. Not sure if that will sway your decision any, just thought I'd mention it.