head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Pure Mechanics
Finger speed and accuracy against fixed note charts of 80s tracks.

Execution · Pure Mechanics
Series peak whose hardest charts became a benchmark for human finger speed.
Guitar Hero Encore: Rocks the 80s leans Execution (Pure Mechanics); Guitar Hero III: Legends of Rock leans Execution (Pure Mechanics).
Guitar Hero III: Legends of Rock leans a little harder on execution, though the gap is small.
Guitar Hero Encore: Rocks the 80s
Hitting note charts at speed is a pure motor ceiling
Guitar Hero III: Legends of Rock
Through the Fire and Flames defines a hands ceiling
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero Encore: Rocks the 80s
No opponent, just the chart
Guitar Hero III: Legends of Rock
Charts are fixed; boss battles are gimmicks
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero Encore: Rocks the 80s
Charts are fixed and fully memorisable
Guitar Hero III: Legends of Rock
Deterministic note charts
Guitar Hero Encore: Rocks the 80s leans a little harder on optimization, though the gap is small.
Guitar Hero Encore: Rocks the 80s
Star power pathing squeezes out extra score
Guitar Hero III: Legends of Rock
Squeeze paths and star power optimization for scores
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.