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
NES platformer that codified boss-weakness ordering and tight jump-and-shoot play.

Execution · Practiced Mastery
Arcade run-and-gun testing dodging execution and full memorisation of fixed spawn patterns.
Mega Man 2 leans Execution (Practiced Mastery); Metal Slug 2 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 2
precise jumps and boss dodges under pressure
Metal Slug 2
Bullet dodging and precise shooting under constant pressure
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man 2
robot masters follow strict patterns
Metal Slug 2
Every enemy and boss is a fixed arcade script
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man 2
deterministic once stage layouts known
Metal Slug 2
Spawns are deterministic; nothing hidden after one credit
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man 2
weapon-weakness ordering is the classic solve
Metal Slug 2
One-credit clears come from memorising every spawn and pattern
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.