Pinsirite or Glalitite, that's the question.
Let's compare them
Pinsirite provides a Dragon/Flying type, which means Ground immunity, Electric neutrality, Ice double weakness (rip), Rock weakness and (lol) double resistance to Grass. He keeps its Dragon and Fairy weaknesses.
It gives a Flying-type Extreme Speed, which is obviously very important and a powerful Flying-type Return.
Losing its Ground type means that Thousand Arrows is no longer STABed
Pinsirite gives +30 Attack and solid bulk with +20 Defnse and +20 Special Defense. +20 Speed means that he outspeeds almost everything at +1.
In conclusion, Pinsirite Zygarde is a bulky Set Up Sweeper and an excellent -Ate speed user. But even with an immunity to Ground, he's weak to EdgeQuake coverage so he can't set up against Offensive PDon.
Here is the classic set of Pinsirite Zygarde
Zygarde @ Pinsirite
Ability: Aura Break
EVs: 252 Atk / 4 Def / 252 Spe
Jolly / Adamant Nature
- Extreme Speed
- Thousand Arrows
- Outrage / Return / Substitute
- Dragon Dance
Glalilite provides an Ice-type Extreme Speed, which is always useful. It also gives an Ice-type Return.
Zygarde keeps its Dragon / Ground type, so ESpeed and Return are not STABed. But in counterpart, he keeps its STABed Thousand Arrows, meaning that ESpeed and Return are less powerful but Thousand Arrows hit PDon and Magearna hard.
Glalite gives +40 Attack (10 more than Pinsirite) so it's... a lot. It also gives a +20 Speed.
It means that Glalilite gives more Attack but no (Special) Defense.
In conclusion, Glalitite provides more Attack and keeps Ground-Type for powerful Thousand Arrows, but have weaker ESpeeds.
He's less bulky but hits harder than Pinsirite Zygarde.
Here is the classic set of Glalitite Zygarde
Zygarde @ Glalitite
Ability: Aura Break
EVs: 252 Atk / 4 Def / 252 Spe
Jolly / Adamant Nature
- Extreme Speed
- Thousand Arrows
- Outrage / Substitute
- Dragon Dance
Conclusion
Pinsirite Zygarde have a more powerful ESpeed and Return. He's also bulkier than Glalitite Zygarde.
Glalilite Zygarde hits harder with Thousand Arrows but have a weaker ESpeed. He also have +10 Attack.
If you want a bulky sweeper, then use Pinsirite Zygarde.
If you want an Ice-type ESpeed and a fast, hard-hitting sweeper, then use Glalitite Zygarde