head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare III demands more execution; StarCraft demands more optimization.

Execution · Grand Arena
Fast arena shooter with deep loadout tuning.

Optimization · Grand Arena
A real-time strategy classic where perfected build orders and mechanical speed decide most matches.
Call of Duty: Modern Warfare III leans Execution (Grand Arena); StarCraft leans Optimization (Grand Arena).
Call of Duty: Modern Warfare III leans clearly harder on execution.
Call of Duty: Modern Warfare III
Fast TTK punishes any aim error
StarCraft
APM and micro control ceiling is huge
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare III
Sliding and pre-aim duels reward prediction
StarCraft
scouting and predicting human opponent builds
Call of Duty: Modern Warfare III leans clearly harder on variance.
Call of Duty: Modern Warfare III
Spawn flips and map traffic add noise
StarCraft
minimal hidden info beyond fog of war
StarCraft leans clearly harder on optimization.
Call of Duty: Modern Warfare III
Loadouts, perks, attachments heavily tuned
StarCraft
build orders and economy are near-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.