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
There is no secret Pokemon in diamond and pearl but there are ways to get Pokemon you cant catch on diamond and pearl. For example manaphy. You must clear the manaphy mission on Pokemon ranger. Goodluck with the game!