head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Pure Mechanics
Comic-panel beat 'em up notorious for punishing health drain and tough fights.

Execution · Pure Mechanics
Memorising fixed spawn patterns and executing them flawlessly under one-hit death.
Comix Zone leans Execution (Pure Mechanics); Metal Slug XX 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.
Comix Zone
Brutal one-life brawling demands clean play
Metal Slug XX
Dodging dense fire one-hit-death demands precise hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Comix Zone
Enemies follow fixed attack scripts
Metal Slug XX
Every enemy and boss is a fixed scripted pattern
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Comix Zone
Everything is placed and scripted
Metal Slug XX
Levels never change between runs
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Comix Zone
Panel routes and item use are memorizable
Metal Slug XX
Memorising spawns and weapon routes is the mastery path
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.