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 · Pure Mechanics
A fast-paced military shooter where twitch aim against other players is the core test.

Execution · Competitive PvP
A classic multiplayer deathmatch shooter distilled to pure arena combat.
Call of Duty: Modern Warfare 3 leans Execution (Pure Mechanics); Deathmatch Classic leans Execution (Competitive PvP).
Call of Duty: Modern Warfare 3 leans a little harder on execution, though the gap is small.
Call of Duty: Modern Warfare 3
aim and reflexes decide most engagements
Deathmatch Classic
aim and movement decide every fight
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 3
predicting human opponent positioning and habits
Deathmatch Classic
human opponents predict positioning and timing
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 3
spawn placement and loadout drops vary
Deathmatch Classic
weapon and spawn randomness
Call of Duty: Modern Warfare 3 leans a little harder on optimization, though the gap is small.
Call of Duty: Modern Warfare 3
loadout tuning and map knowledge help
Deathmatch Classic
limited map and weapon-control 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.