head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Tony Hawk's Underground 2 demands more execution.

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

Execution · Practiced Mastery
Combo execution layered over solving each park's optimal scoring line.
Grand Theft Auto V leans Execution (Practiced Mastery); Tony Hawk's Underground 2 leans Execution (Practiced Mastery).
Tony Hawk's Underground 2 leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Tony Hawk's Underground 2
Combo timing, balance meters and manuals demand practiced hands
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 2
No opponent adapts; score attack is solo
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Tony Hawk's Underground 2
Maps and goals are fixed and fully knowable
Tony Hawk's Underground 2 leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Tony Hawk's Underground 2
Finding the highest-scoring line through each park is 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.