4th Gen Diamond or Platinum?

[groudonlover]

If you don't want platinum just because Palkia is too Pink, just ignore it!
You can get both Dialga and Palkia AFTER the National Dex, and in that order.
If you don't want Palkia, just catch Dialga and ignore Palkia.
You need to talk to the elder in Celestic Town after the National Dex, and she will tell you something, if you then go to Spear Pillar, Dialga will first be there, and then if you repeat, there will be Palkia.

The Distortion World is an incredibly cool but sightly annoying the first time place, where Giratina is.
If you want to know the whole story, click the Spoiler:

Spoiler:

When you beat the Tag Battle between You and your Rival vs Saturn and Mars, Cyrus will use the Red Chain to summon Dialga, and then an artificial copy to summon Palkia.
Just like in D/P, Uxie, Mesprit and Azelf comes to balance out the whole thing, but they can only keep one of them in balance at a time, something that Cyrus knew, and therefore summoned both of them to prevent the Lake Trio from ruining his plan.
Just when he thinks he has succeeded in creating a new world, Several black spots, like spilled Ink, appears on the floor in front of him.
The ink blobs floats together in a round hole, in which you can see Giratina's eyes and the red claws on it's wings.
Cyrus babbles a lot about a shadow and a reverse reality, and then says "I won't have any more infefefefefe.... aaaaaah!" when giratina grabs him and drags him into the hole, leaving after it a glowing, circular Portal to the Distortion World, where you must follow Cynthia, who so badly wich to rescue her little Cyrus(not really) and then you must travel downwards to the bottom of it, where you will finally battle Cyrus, who tries desperately to fool you, and says that if you capture Giratina, both worlds will be destroyed.
Cynthia throws his lies aside, and tells you to prove to Giratina how badly wou want it to join you.
When you have caught/defeated Giratina, you jump trough another portal to get back to the normal world, more exactly to Sendoff Spring.

 

[z4pd0s]

get platinum, hands down. its the two versions combined and more stuff. why only get half the pokemon when you can catch em all? :)

 

[Niprop]

...Why choose when you can get em both?

In all seriousness though, I'd have to say Platinum, if only because you have a bit more to do at the end of the main game.