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
The Source-engine era of Counter-Strike's aim-and-economy tactical formula.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Counter-Strike: Source leans Execution (Competitive PvP).
Counter-Strike: Source 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
Counter-Strike: Source
Recoil control and crosshair placement decide duels
Counter-Strike: Source leans a little harder on reads, though the gap is small.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Counter-Strike: Source
Outguessing rotations, fakes, and timing of human opponents
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 2
Spawn flips and killstreak randomness
Counter-Strike: Source
Hidden enemy positions until information is gathered
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 2
Killstreak economy, loadout and map knowledge
Counter-Strike: Source
Economy rounds, utility lineups, and map control theory
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.