Facts first? Palkia only has a 2x weakness to Dragon.
Dialga have a 2x weakness to Ground (On noez, EQ!) and Fighting, meaning that it might be easier to knock out.
But in a Dialga vs Palkia battle, I think that Dialga would win. Super effective Roar of Time with STAB is way better than a STAB neutral Spacial Rend, or so I think. So for me?
Dialga, without doubt. 11 resistances, one immunity, and a powerhouse attack. Palkia, goodbye.