"Stronger" is the most relative term you can use in any RPG, so I'll say right now, that neither Dialga nor Palkia are "stronger" than the other.
It totally depends on what you're comparing them in. Palkia is faster than Dialga by 10 points, Palkia has less bulk, and learns Spacial Rend over Roar of Time. Palkia is clearly supposed to be looked at as a Mixed Sweeper, ripping holes through other enemies. It has a fair typing (Only one weakness!), but it doesn't bear many resistances. Because of this, Palkia can't survive as long as Dialga can.
Dialga has a near-perfect typing, bearing an immense amount of resistances, has lower Speed, but more HP, and learns slightly weaker attacks, that are more used to make sweeping easier in the future. Dialga is obviously supposed to be more of a tank, that battles slowly, but knocks through the wreckage easily, whereas Palkia can't.
Like I said, even though the two have nearly identical movepools and stats, they are used for extremely different roles. If by "stronger," you mean which hits harder, then I'd say Palkia. If by "Stronger," you mean the ability to survive, then I'd say Dialga.
The two are very different, so I think its unfair to identify which one is actually better.