I've always gone for the version that comes first when each Pokémon game came out, so I got Red, Gold and Ruby. However, I stopped doing this when FireRed and LeafGreen came out and bought LeafGreen. I'm buying both versions this time and giving the Diamond version to my brother. Diamond probably has the cooler Legendary Pokémon but they're both equal really (unlike Groudon and Kyogre, when put against each other, Kyogre always wins). They are both basically the same as each other but with different Pokémon.