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

Execution · Practiced Mastery
Pattern memorisation and precise platforming with a solvable boss-order route.

Execution · Practiced Mastery
Twitch dodging and shooting layered over memorised arcade stage routes.
Mega Man 5 leans Execution (Practiced Mastery); Metal Slug leans Execution (Practiced Mastery).
Metal Slug leans a little harder on execution, though the gap is small.
Mega Man 5
Precise jumps and buster timing through gauntlet stages
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 5
Robot Masters 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 5
Every stage and boss is identical each attempt
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 5
Weapon-order routing and pattern memorisation solve the game
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.