I have to go with Platinum. The game felt slightly faster than Diamond and Pearl, especially with the improved surfing speed, which I'm unsure of why they had nerfed it from Gen 3 in the first place. I also liked how Giratina had been given a major role in the story, but they didn't fully scrap Dialga and Palkia from the story. Even better was the ability to catch both the legendaries instead of only being able to get one and have to trade to get the other. The addition of the other Rotom forms was really cool and it was the first step towards making Rotom a much more useful Pokemon in the forms of Rotom Wash and Rotom Heat. The Battle Frontier's return from Gen 3 was a nice touch as well, but I honestly still like Emerald's frontier over Gen 4's version. The story felt a bit better, the Distortion World was a cool addition along with Origin Form Giratina, and the larger variety of Pokemon was cool and made the game a bit more difficult. Platinum for sure is the superior game in my opinion.