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

Execution · Grand Arena
Fast arena shooter with deep loadout tuning.

Execution · Pure Mechanics
Pure aim and movement skill tested against adaptive human opponents competing for spawn-cycle dominance.
Call of Duty: Modern Warfare III leans Execution (Grand Arena); Quake III Arena leans Execution (Pure Mechanics).
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 III
Fast TTK punishes any aim error
Quake III Arena
Rail and rocket aim at speed is the entire game
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 III
Sliding and pre-aim duels reward prediction
Quake III Arena
Human opponents adapt their routes and timings in direct response
Call of Duty: Modern Warfare III leans a little harder on variance, though the gap is small.
Call of Duty: Modern Warfare III
Spawn flips and map traffic add noise
Quake III Arena
Spawn locations have mild randomness but are quickly mapped
Call of Duty: Modern Warfare III leans a little harder on optimization, though the gap is small.
Call of Duty: Modern Warfare III
Loadouts, perks, attachments heavily tuned
Quake III Arena
Armor and weapon timing routes are learnable and highly leveraged
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.