I would wrap up Pearl, and then jump to X/Y.
In 4th generation, Platinum is basically the same thing as Diamond/Pearl but with a few extra additions (similar to how Emerald is for Ruby/Sapphire.) Personally, I don't think it's worth playing through Platinum if you already played Pearl.
Did you ever play Gold/Silver? Heartgold and Soulsilver are remakes of those games. I think you could skip these games. If you were to choose to play either Platinum or HG/SS, I'd say play HG/SS, but still you could skip these.
Generation 5 made a huge change. Whereas games like Platinum and Emerald are just D/P and R/S with modifications, Black and White 2 are sequels to Black and White. Remember how in Gold/Silver, you visited Kanto 2 years later to find how it changed? That's kind of what Black and White 2 does. Many people predict that Generation 6 (X/Y) might do the same thing.
So I'd say either: Finish Pearl (4th gen), jump straight to X/Y (6th gen)
or: Finish Pearl (4th gen), play Black/White2 (5th gen), and then play X/Y (6th)
It kinda depends on how much free time you have to play pokemon. Sabrewulf made a very insightful point, that Emerald/Pearl pokemon cannot be transfered to 6th gen until they're transfered to 5th gen. (But then there's the possibility to consider that a friend could let you borrow their 5th gen game, purely for transfering purposes.)