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 · Competitive PvP
A tactical shooter where precise gunplay against human opposition is the deciding skill.

Execution · Pure Mechanics
Pure aim and movement skill tested against adaptive human opponents competing for spawn-cycle dominance.
Counter-Strike: Condition Zero leans Execution (Competitive PvP); Quake III Arena leans Execution (Pure Mechanics).
Quake III Arena leans a little harder on execution, though the gap is small.
Counter-Strike: Condition Zero
aim and spray control decide duels
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.
Counter-Strike: Condition Zero
predicting human opponent positioning
Quake III Arena
Human opponents adapt their routes and timings in direct response
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Counter-Strike: Condition Zero
buy-round economy and drops vary slightly
Quake III Arena
Spawn locations have mild randomness but are quickly mapped
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Counter-Strike: Condition Zero
economy management and utility usage solvable
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.