This all depends on what you like better. If you like a steel-dragon for diamond or water-dragon for pearl. Though they both do have all the same pokemon except for the two dragons. But it all comes down to what you like.
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diamond
neither of them is better, they are both the same, but in diamond, you catch dialga, and in pearl, you catch palkia, and you can catch different Pokemon in each game for example: you can catch this Pokemon in diamond, but not pearl but: you can catch this different Pokemon in pearl, but not diamond
diamond. the legendary has a better special move, and you can get a tyrannitar on there without importing.
I believe that Pokemon platinum is better than diamond and pearl because it offers more features, and allows you to do much more cheats and tricks and get more Pokemon through normal game play than Pokemon diamond and pearl. Yes, it is.
pearl is better even though i play diamond because it has better Pokemon ans certifiable and that is my finally answer.