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 · Pure Mechanics
A rail-shooter score test of raw accuracy over fully memorised spawn patterns.
Metal Slug leans Execution (Practiced Mastery); The House of the Dead: Remake leans Execution (Pure Mechanics).
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 House of the Dead: Remake
An aimbot trivialises it - accuracy and reload timing are everything
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 House of the Dead: Remake
Zombies spawn on rails, never adapting
The House of the Dead: Remake leans a little harder on variance, though the gap is small.
Metal Slug
Spawns are nearly deterministic once a stage is learned
The House of the Dead: Remake
Branching paths and item spots are quickly mapped
Metal Slug leans a little harder on optimization, though the gap is small.
Metal Slug
One-credit clears come from memorising routes and spawn patterns
The House of the Dead: Remake
Memorising spawn orders and 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.