Honestly, Giratina would be the better choice in this scenario.
Why? well, let's see...
Giratina boasts a base 150 HP stat, with Base 120 Defenses.
Palkia can only show he has a Base 90 HP stat, with a Base 100 Defense, and a base 120 SDefense.
Clearly, you can see that Giratina can take a hit much easier, so I am suggesting you use it for that purpose.