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
Twitch dodging and shooting layered over memorised arcade stage routes.

Execution · Practiced Mastery
Arthur's infamous SNES gauntlet of armor-shattering hazards that must be beaten twice with the right weapon.
Metal Slug leans Execution (Practiced Mastery); Super Ghouls 'n Ghosts 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 Ghouls 'n Ghosts
Committed double jump and relentless hazards punish errors
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 Ghouls 'n Ghosts
Scripted spawns, no adaptation
Super Ghouls 'n Ghosts leans a little harder on variance, though the gap is small.
Metal Slug
Spawns are nearly deterministic once a stage is learned
Super Ghouls 'n Ghosts
Chest contents and zombie spawns add light randomness
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 Ghouls 'n Ghosts
Weapon management and mandatory second loop demand routing
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.