head to head
Neither is simply harder - they are hard in different places. Guitar Hero III: Legends of Rock demands more execution; Super Mario Odyssey demands more optimization.

Execution · Pure Mechanics
Series peak whose hardest charts became a benchmark for human finger speed.

Execution · Pure Mechanics
A 3D platformer whose joy lives in tight movement and capture-driven traversal.
Guitar Hero III: Legends of Rock leans Execution (Pure Mechanics); Super Mario Odyssey leans Execution (Pure Mechanics).
Guitar Hero III: Legends of Rock leans clearly harder on execution.
Guitar Hero III: Legends of Rock
Through the Fire and Flames defines a hands ceiling
Super Mario Odyssey
Precise platforming and capture-move tech ceiling.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero III: Legends of Rock
Charts are fixed; boss battles are gimmicks
Super Mario Odyssey
No adaptive opponents whatsoever.
Super Mario Odyssey leans a little harder on variance, though the gap is small.
Guitar Hero III: Legends of Rock
Deterministic note charts
Super Mario Odyssey
Almost no randomness in level design.
Super Mario Odyssey leans clearly harder on optimization.
Guitar Hero III: Legends of Rock
Squeeze paths and star power optimization for scores
Super Mario Odyssey
Moon routes and movement tech learnable.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.