head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto: San Andreas demands more variance; Tony Hawk's Underground demands more execution.

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

Execution · Practiced Mastery
A story-driven skateboarding game still won by trick execution.
Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery); Tony Hawk's Underground leans Execution (Practiced Mastery).
Tony Hawk's Underground leans clearly harder on execution.
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Tony Hawk's Underground
Combos plus off-board tricks keep the input ceiling high
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Tony Hawk's Underground
Story missions are scripted, not adaptive
Grand Theft Auto: San Andreas leans clearly harder on variance.
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Tony Hawk's Underground
Fixed levels and mission scripts
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
Tony Hawk's Underground
Mission and score-line routing remain solvable
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.