Your logic confuzzles me. Diamond and Pearl are essentially the same games with few differences in their Pokemon. The way you word makes it sounds like they're two completely different games when both of them have the same dungeons, items, and locales.
Anyways, I'd say Platinum simply becuase it's D/P with way more features.
I wish I could vote neither. The fourth generation was by far my least favorite, but if I had to, I'd vote Platinum because it has all of the updated features, and the Distortion World was pretty cool.