Giant-Crusher is a warhammer; Greatsword is a colossal sword. Both are pure Strength monsters. The choice comes down to moveset preference and specific advantages each weapon offers.
Giant-Crusher
Maximum stagger and single-hit damage
Strengths
- Highest poise damage of any weapon
- Strike damage type is effective vs armored enemies
- Charges Royal Knight's Resolve for one-shot potential
- Great jump attacks
Weaknesses
- Very slow attack speed
- Heavy weight requires high Endurance
- Short reach compared to Greatsword
- Punishing if you miss
VS
Greatsword
PvP and versatile strength gameplay
Strengths
- Crouch poke is the fastest colossal attack in PvP
- Available within 30 minutes of starting
- Better reach than most colossal weapons
- Slash/thrust moveset covers more situations
Weaknesses
- Slightly less poise damage than Giant-Crusher
- Standard slash damage less effective vs armor
- No innate special properties
- Just a big sword (some find it boring)
Verdict
Giant-Crusher wins for PvE bossing with its superior stagger and one-shot potential. Greatsword wins for PvP with the infamous crouch poke. Both are excellent; pick based on whether you prioritize boss killing or player killing.