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
Precise platforming execution over a solvable boss-order and upgrade-routing puzzle.

Execution · Practiced Mastery
Twitch dodging and shooting layered over memorised arcade stage routes.
Mega Man X2 leans Execution (Practiced Mastery); Metal Slug 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 X2
Dash-jump precision and boss dodge timing define clears
Metal Slug
Dodging dense bullet patterns and precise shots carry every credit
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X2
Mavericks run fixed attack scripts, never adapting
Metal Slug
Enemies follow fixed arcade scripts, never adapt
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X2
Everything is deterministic once stages are seen
Metal Slug
Spawns are nearly deterministic once a stage is learned
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mega Man X2
Boss weakness order and upgrade routing are solvable
Metal Slug
One-credit clears come from memorising routes and spawn patterns
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.