head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Execution · Pure Mechanics
A fast-paced military shooter where twitch aim against other players is the core test.

Execution · Competitive PvP
Hardcore tactical shooter with lethal TTK.
Call of Duty: Modern Warfare 3 leans Execution (Pure Mechanics); Insurgency: Sandstorm leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 3
aim and reflexes decide most engagements
Insurgency: Sandstorm
One-shot kills demand pixel-perfect aim
Insurgency: Sandstorm leans a little harder on reads, though the gap is small.
Call of Duty: Modern Warfare 3
predicting human opponent positioning and habits
Insurgency: Sandstorm
Holding angles and pushes are mind games
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 3
spawn placement and loadout drops vary
Insurgency: Sandstorm
Very low RNG, near pure skill
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 3
loadout tuning and map knowledge help
Insurgency: Sandstorm
Loadout points and class roles tuned
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.