Giratina. With it, you get the Griseous Orb, which can't be traded over from Platinum to Hg/SS. You can trade Lustrous/Adamant Orb's to my knowledge.
EDIT: Battle wise, each has it's own pros and cons. Palkia is the fastest between them, and outspeeds a fair bit in the Uber metagame. Giratina has a lot of bulk for a sweeper, and ghost immunity makes it the best offensive anti-spinner, not to mention it has priority (shadow sneak). Dialga, with it's Steel typing, makes a fantastic tank.