It really depends on what you're looking for because Palkia and Dialga are good in their own different ways. Dialga itself has great amounts of defense, lower speed, but more HP, and makes sweeping a little easier on the team. SO, he is basically a huge tank. On the other hand, Palkia is a little bit faster, has not as much "tank" but can triumph with his Spacial Rend compared to Dialga's Roar of Time, and is seen more as a mixed sweeper with good typing of mainly one weakness. But this also means he can't survive as long and is mainly used for hitting huge power towards its' enemy. Dialga isn't very fast but is capable of taking more hits and does what Palkia can't. SO in a nutshell they aren't comparable.
If you want huge power, it's Palkia. But if you're all about defense and want to survive longer to wear out the enemy, it's Dialga.
Gummayberz~ FC: 2595-0819-5240; IGN: Iceprinces; Rock Safari: Corsola, Nosepass, and Barbaracle; Breeder