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

Execution · Practiced Mastery
Punishing platforming execution over learnable boss orders and part builds.

Execution · Practiced Mastery
Arthur's infamous SNES gauntlet of armor-shattering hazards that must be beaten twice with the right weapon.
Mega Man X6 leans Execution (Practiced Mastery); Super Ghouls 'n Ghosts leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X6
Cruel platforming and boss windows demand frame-tight inputs
Super Ghouls 'n Ghosts
Committed double jump and relentless hazards punish errors
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X6
Bosses are fixed pattern sets
Super Ghouls 'n Ghosts
Scripted spawns, no adaptation
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X6
The Nightmare System shuffles level hazards between runs
Super Ghouls 'n Ghosts
Chest contents and zombie spawns add light randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X6
Weapon orders, armor parts and route knowledge are essential
Super Ghouls 'n Ghosts
Weapon management and mandatory second loop demand routing
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.