I've reviewed Palkia's and Dialga's stats. Palkia is more of an offensive Pokemon than Dialga since it trades HP for Speed in its stats. The larger difference is that Dialga has a larger Defense stat while Palkia has a larger Special Defense stat.
What really caught my attention was the number of resistances that Dialga has. It is resistant against 12 out of 17 types, one being an immunity. To clearify, Dialga has two weaknesses to Fighting and Ground. The problem is that Fighting and Ground are two very common types, especially in ubers. It doesn't have the speed to back itself up, either. The good part is that Dialga boasts in Defense, which Fighting and Ground are usually parallel too. Dialga can hang on when attacked by its weak points.
Now, on Palkia, there are many differences and similarities. A big difference is that Palkia only has three resistances, compared to Dialga's twelve. That is a setback, however, Palkia does have less weaknesses, and slightly less common than a ground weakness. Palkia is superior to Dialga in its Speed stat, allowing it to outrun somewhat of a number of Pokemon. Manaphy, Mew, Kyogre, Groudon, Rayquaza, etc. are all outran if Palkia has the right EVs and IVs pumped into it.
In my opinion, they can rival eachother very well. But my vote shall go to Palkia.