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
Twitch dodging and shooting over fully memorisable enemy wave patterns.
Metal Slug leans Execution (Practiced Mastery); Star Soldier 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
Star Soldier
Dodging dense fire and rapid shooting carry every run
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
Star Soldier
Enemy waves are identical scripted formations
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
Star Soldier
Nothing random once spawn patterns are learned
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
Star Soldier
Memorising waves and power-up routes defines high scores
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.