head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Execution · Pure Mechanics
WWII gunfight execution with light multiplayer prediction.

Execution · Practiced Mastery
Slow-motion gunfights against aggressive but ultimately mappable squad AI.
Call of Duty 3 leans Execution (Pure Mechanics); F.E.A.R. 3 leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty 3
Aimbot trivialises both campaign and multiplayer gunfights
F.E.A.R. 3
Slow-mo gunplay still rewards fast accurate aim
Call of Duty 3 leans a little harder on reads, though the gap is small.
Call of Duty 3
Multiplayer adds modest human positional prediction
F.E.A.R. 3
Squad AI flanks convincingly but follows mappable routines
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty 3
Scripted setpieces and spawn flow add minor unknowns
F.E.A.R. 3
Encounter layouts are scripted-but-unknown first time
F.E.A.R. 3 leans a little harder on optimization, though the gap is small.
Call of Duty 3
Map knowledge and weapon choice reward light study
F.E.A.R. 3
Cover usage and power timing are quickly learned
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.