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

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
A story-driven skateboarding game still won by trick execution.
Grand Theft Auto V leans Execution (Practiced Mastery); Tony Hawk's Underground leans Execution (Practiced Mastery).
Tony Hawk's Underground leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Tony Hawk's Underground
Combos plus off-board tricks keep the input ceiling high
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Tony Hawk's Underground
Story missions are scripted, not adaptive
Grand Theft Auto V leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Tony Hawk's Underground
Fixed levels and mission scripts
Tony Hawk's Underground leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
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.