head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Pure Mechanics
The last mainline Guitar Hero, wrapping its setlist in a quest with score power-ups.

Execution · Pure Mechanics
The full-band rhythm game whose setlist and feel made it the genre's high-water mark.
Guitar Hero: Warriors of Rock leans Execution (Pure Mechanics); Rock Band 2 leans Execution (Pure Mechanics).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero: Warriors of Rock
Expert charts and the brutal 2112 finale
Rock Band 2
Guitar, drum, and vocal accuracy on expert charts
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero: Warriors of Rock
Nothing adapts
Rock Band 2
Charts never respond
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero: Warriors of Rock
Deterministic charts
Rock Band 2
Fixed setlist charts
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Guitar Hero: Warriors of Rock
Power-up choices and star power routing
Rock Band 2
Overdrive activation paths decide leaderboard 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.