This is really a matter of opinion. The two versions vary on which Pokemon you can catch.
Many think Diamond is the better game because of the better looking Pokemon and Dialga looks stranger then Palkia, but, in true fact, Pearl is a better version. Plakia is the stronger one. Dalga is Dragon and Steel type, where as Palkia is Dragon and Water. The two dragon types cancle each other out, but water has a better effect on steel then steel has on water, but neither are super effective to each other.
Another reason why Pearl is better is because the Pokemon seem to level up quicker then on Diamond. I have both games and I have had a Dratini on each, lv 1. Both of them battled a Starly lv 2 and the Pearl Dratini's exp level was a few sections higher then the Diamond Dratini when they both got the same amount of exp.
The final reason why Pearl is better is because of Poffins and Amurty Square. You have a better chance of getting a good Poffin on Pearl then you do on Diamond. My friend and I found this out when I made a Poffin on both games, same berry, same way, no burns not spillage and the one on Pearl was better. And Amurty Square, you have a much better chance of getting a good item there then you do on Diamond.
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THE POKEMON THAT ARE NOT FOUND IN DIAMOND AND PEARL ARE:MANAPHY PHIONE
You can get Palkia in Pearl, otherwise you have to transfer Palkia from Pokemon Pearl to Pokemon Diamond.
some of the Pokemon are different such as diamond has dialga and pearl has palkia some of the Pokemon are different such as diamond has dialga and pearl has palkia