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
Twitch dodging and shooting layered over memorised arcade stage routes.

Execution · Practiced Mastery
Memorising spawn patterns then executing them flawlessly under one-hit-death rules.
Metal Slug leans Execution (Practiced Mastery); Super Contra 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.
Metal Slug
Dodging dense bullet patterns and precise shots carry every credit
Super Contra
One-hit deaths demand precise dodging and constant accurate fire
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Metal Slug
Enemies follow fixed arcade scripts, never adapt
Super Contra
Alien waves follow fixed spawn scripts
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Metal Slug
Spawns are nearly deterministic once a stage is learned
Super Contra
Nothing random once spawn patterns are memorised
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Metal Slug
One-credit clears come from memorising routes and spawn patterns
Super Contra
Memorising spawns and weapon routes is half the run
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.