There are no cheats without action replay for pearl but there are glitches but if some of them require for you to do the Pokemon trading thing do NOT do them because the Pokemon company can ban you from trading. Just search Pokemon pearl glitches I know there's tons of them.
You can't get diagla in Pokemon pearl without trading but the way to get him in the pearl pokedex is by going to celestic town and going into the big house and talk to the old lady. Only AFTER you beat the Elite four.
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