head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto: San Andreas demands more optimization; WCW Mayhem demands more reads.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.

Execution · Practiced Mastery
Timing-based grapple execution with light opponent prediction in multiplayer matches.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); WCW Mayhem leans Execution (Practiced Mastery).
WCW Mayhem leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
WCW Mayhem
Timing grapples, reversals and strikes decides every match
WCW Mayhem leans clearly harder on reads.
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
WCW Mayhem
Human opponents in versus add a modest prediction layer
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
WCW Mayhem
Weapon spawns and AI behaviour add minor noise
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
WCW Mayhem
Learning move lists and match-type quirks is a fixed problem
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.