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
Large-scale modern combined-arms shooter built on destructible maps and squad teamwork.

Execution · Competitive PvP
Blockbuster 2009 sequel famous for killstreak chains and frantic multiplayer.
Battlefield 3 leans Execution (Competitive PvP); Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP).
Call of Duty: Modern Warfare 2 leans a little harder on execution, though the gap is small.
Battlefield 3
Gunplay, suppression management, and vehicle piloting
Call of Duty: Modern Warfare 2
Fast time-to-kill rewards raw aim and reflex
Call of Duty: Modern Warfare 2 leans a little harder on reads, though the gap is small.
Battlefield 3
Human teams adapt routes and armor pushes
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Battlefield 3
Spawns, destruction, and vehicle drops shuffle engagements
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Battlefield 3
Unlock paths, class synergy, and map control
Call of Duty: Modern Warfare 2
Killstreak economy, loadout and map knowledge
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.