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 · Competitive PvP
Arcade punch timing with counter prediction in human bouts.
Call of Duty 3 leans Execution (Pure Mechanics); Ready 2 Rumble Boxing 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 3
Aimbot trivialises both campaign and multiplayer gunfights
Ready 2 Rumble Boxing
Punch timing, dodges and rumble combos reward quick hands
Ready 2 Rumble Boxing leans a little harder on reads, though the gap is small.
Call of Duty 3
Multiplayer adds modest human positional prediction
Ready 2 Rumble Boxing
Human bouts become feint-and-counter prediction
Call of Duty 3 leans a little harder on variance, though the gap is small.
Call of Duty 3
Scripted setpieces and spawn flow add minor unknowns
Ready 2 Rumble Boxing
Nothing hidden once boxers are learned
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty 3
Map knowledge and weapon choice reward light study
Ready 2 Rumble Boxing
Boxer matchups and rumble meter use are quickly mapped
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.