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
Routing and executing perfect scoring runs through scripted shooting arenas.
Metal Slug leans Execution (Practiced Mastery); The Club 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
The Club
Chaining kills without dropping combo demands fast precise aim
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
The Club
Enemy spawns are scripted and memorisable
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
The Club
Nothing random once spawn layouts 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
The Club
Score routes through each arena are the real puzzle
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.