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
Combined-arms multiplayer shooter famous for map-altering levolution moments and 64-player conquest.

Execution · Competitive PvP
Blockbuster 2009 sequel famous for killstreak chains and frantic multiplayer.
Battlefield 4 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 4
Gunplay, vehicle mastery, and movement under fire
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 4
Human teams adapt to flanks and levolution chokes
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 4
Map-altering events and spawns inject noise
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 4
Loadout tuning and objective rotation knowledge
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.