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

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

Execution · Practiced Mastery
Tests arcade grapple timing with shallow systems underneath.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); WCW Nitro leans Execution (Practiced Mastery).
WCW Nitro leans a little harder on execution, though the gap is small.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
WCW Nitro
Fast grapple timing in an arcade-paced wrestling frame
WCW Nitro leans clearly harder on reads.
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
WCW Nitro
Human versus play rewards mild move prediction
Grand Theft Auto: San Andreas leans a little harder on variance, though the gap is small.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
WCW Nitro
Little randomness beyond AI behaviour
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 Nitro
Learning each wrestler's move list is quickly exhausted
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.