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

Execution · Competitive PvP
Blockbuster 2009 sequel famous for killstreak chains and frantic multiplayer.

Execution · Competitive PvP
Hardcore tactical shooter with lethal TTK.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Insurgency: Sandstorm leans Execution (Competitive PvP).
Insurgency: Sandstorm leans a little harder on execution, though the gap is small.
Call of Duty: Modern Warfare 2
Fast time-to-kill rewards raw aim and reflex
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 2
Outguessing human rotations and flanks
Insurgency: Sandstorm
Holding angles and pushes are mind games
Call of Duty: Modern Warfare 2 leans a little harder on variance, though the gap is small.
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Insurgency: Sandstorm
Very low RNG, near pure skill
Call of Duty: Modern Warfare 2 leans a little harder on optimization, though the gap is small.
Call of Duty: Modern Warfare 2
Killstreak economy, loadout and map knowledge
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.