Whichever suits your team. If you're playing Trick Room or bulky offense, then use Glastrier. If you're playing offense, use Spectrier.
Most offensive Spectrier use Dynamax, as its mediocre Special Attacks like Hyper Beam, Mud Shot, and Dark Pulse get a boost and a cool secondary effect. Some support variants exist too.
Spectrier @ Life Orb
Ability: Grim Neigh
EVs: 76 HP / 248 SpA / 184 Spe
Timid Nature
- Shadow Ball
- Mud Shot
- Hyper Beam
- Dark Pulse / Protect
Although most people think Dynamax Trick Room Weakness Policy when they think of Glastrier, in this unrestricted metagame, Life Orb or Assault Vest are usually better and don't require as much support.
Glastrier @ Life Orb
Ability: Chilling Neigh
EVs: 252 HP / 228 Atk / 16 Def / 12 SpD
Adamant Nature
- Icicle Crash
- High Horsepower
- Protect
- Close Combat
Glastrier @ Assault Vest
Ability: Chilling Neigh
EVs: 252 HP / 252 Atk / 4 SpD
Brave / Adamant Nature
- Icicle Crash
- High Horsepower
- Close Combat
- Heavy Slam