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
Stick execution and opponent reading within football's bounded chaos.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); FIFA 15 leans Execution (Competitive PvP).
Call of Duty: Modern Warfare 2 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
FIFA 15
Dribble and shot timing decide most online matches
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
FIFA 15
Human opponents adapt formations and bait your defending
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
FIFA 15
Deflections and bounce physics swing single matches
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
FIFA 15
Squad building and meta tactics reward dedicated study
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.