head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare 2 demands more variance; Persona 4 Arena demands more reads.

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

Execution · Competitive PvP
Combo execution and endless mixup prediction against adapting human opponents.
Call of Duty: Modern Warfare 2 leans Execution (Competitive PvP); Persona 4 Arena leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2
Fast time-to-kill rewards raw aim and reflex
Persona 4 Arena
Combo execution and confirm timing gate competitive play
Persona 4 Arena leans clearly harder on reads.
Call of Duty: Modern Warfare 2
Outguessing human rotations and flanks
Persona 4 Arena
Mixups and throw-tech guessing against adapting humans never converge
Call of Duty: Modern Warfare 2 leans clearly harder on variance.
Call of Duty: Modern Warfare 2
Spawn flips and killstreak randomness
Persona 4 Arena
Almost nothing random in a fighting match
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
Persona 4 Arena
Frame data, matchup study and meter math reward homework
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.