head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare 2 demands more variance; Mortal Kombat demands more reads.

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

Reads · Competitive PvP
The 2011 reboot that returned Mortal Kombat to brutal 2D fighting fundamentals.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Mortal Kombat leans Reads (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
Mortal Kombat
Combo execution and punish timing
Mortal Kombat leans clearly harder on reads.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Mortal Kombat
Human opponents adapt; conditioning and counter-guessing rule
Call of Duty: Modern Warfare 2 leans clearly harder on variance.
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Mortal Kombat
Near-deterministic systems
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
Mortal Kombat
Frame data and matchup study converge to known answers
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.