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
Tests precise shooting and route optimization in timed score-attack arenas.

Execution · Practiced Mastery
An over-the-top horde shooter throwing hundreds of charging enemies at you.
Resident Evil: The Mercenaries 3D leans Execution (Practiced Mastery); Serious Sam: The Second Encounter 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.
Resident Evil: The Mercenaries 3D
Headshot accuracy and melee-chain timing drive every score run
Serious Sam: The Second Encounter
Circle-strafing huge horde waves tests stamina and aim
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Resident Evil: The Mercenaries 3D
Enemy hordes spawn on fixed score-attack logic
Serious Sam: The Second Encounter
Enemies charge in fixed patterns
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Resident Evil: The Mercenaries 3D
Spawn mixes add slight run-to-run noise
Serious Sam: The Second Encounter
Wave compositions memorised after one run
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Resident Evil: The Mercenaries 3D
Combo routes and time-bonus placement are deeply optimizable
Serious Sam: The Second Encounter
Prioritising targets and managing arenas is solvable
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.