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

Execution · Practiced Mastery
Tight platforming execution against fixed boss patterns with a solvable upgrade route.

Execution · Practiced Mastery
Twitch dodging and shooting layered over memorised arcade stage routes.
Mega Man X3 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 X3
Dash-jump precision and boss dodging demand tight inputs
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 X3
Mavericks cycle fixed attack patterns
Metal Slug
Enemies follow fixed arcade scripts, never adapt
Metal Slug leans a little harder on variance, though the gap is small.
Mega Man X3
Nothing random - every pickup and enemy is placed
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 X3
Weapon-order and upgrade routing is a known solved sequence
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.