Pokemon Platinum is a superior game to Diamond and Pearl in all ways. If you haven't picked up a Pokemon game in a few years, then this is the one to go for. It's nowhere as revolutionary or ahead of its time as Gold/Silver/Crystal, but it's pretty close. Even if you have D/P, Platinum is worth getting for the new story points and catchable Pokemon.
It's also the only way to obtain the new forms for Giratina, Shaymin and Rotom – if you're into that sort of thing. Even six months after thoroughly defeating the Japanese version of this game, Pokemon Platinum remains one of the best RPG's for the DS and a nice way for Pokemon fanss to kill time until the 12th movie and Explorers of the Sky come out.