Since both have the same typing, which one is better?
Primal form Giratina is superior in almost every way, being immune to 8 types and having amazing stats. However, Giratina can’t be used in the Battle Frontier and so it becomes useless if that’s what you’re currently doing. So overall Primal Giratina is the best.
Giratina, without a doubt, has really fantastic bulk, and able to learn a variety of moves, such as Calm Mind, Aura Sphere, etc. that D Charizard cannot have access to. Even if it is just base Giratina, it will still significantly better than D Charizard.
However, if you don’t have the crystal piece or the orb, then I suggest Mega Delta Charizard.
Pretty sure you get the crystal piece upon catching it right? Since you catch it in its Primal form?
You get the peice, but you can’t equip it. You need to wait for the post-game for that.