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
Arcade run-and-gun testing crowd dodging and spawn memorisation.

Execution · Pure Mechanics
Rhythm FPS where every shot must land on the metal soundtrack's beat.
Mercs leans Execution (Pure Mechanics); Metal: Hellsinger 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.
Mercs
Run-and-gun crowd weaving at arcade pace is everything
Metal: Hellsinger
shooting on the beat is the entire skill test
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mercs
Soldiers spawn on fixed scripts
Metal: Hellsinger
demons follow fixed behavior
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mercs
Spawn tables barely vary between credits
Metal: Hellsinger
deterministic arenas
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mercs
Weapon pickups and route memorisation stretch each credit
Metal: Hellsinger
weapon choice and streak management for 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.