If I were you, I'd buy two copies of Platinum. Diamond and Pearl were okay, but Pokémon Platinum was even better—it had tons of improvements over them. It had an extended regional dex, access to Battle Frontier (sort like the one from Emerald, if you've played that game before), additional forms for a few legendaries, and you could get to Distortion World. Distortion World is an awesome place, and once you get there, you'll be totally amazed. There might be nothing in there other than Giratina, but in my opinion, it's a pretty fun place to hang around. Oh, and after you're done the story, you also get to have your own villa at a location known as "Resort Area." You could customize this villa with your own furniture and such, and various people will come visiting you each day. (: There are lots of other things in Platinum that Pokémon Diamond and Pearl don't have, but these are the only things that I could remember right now.