head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; TrackMania United Forever demands more execution.

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

Execution · Practiced Mastery
Pure time-trial racing testing relentless execution of an ever-more-optimised line.
Grand Theft Auto V leans Execution (Practiced Mastery); TrackMania United Forever leans Execution (Practiced Mastery).
TrackMania United Forever leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
TrackMania United Forever
Frame-perfect lines over hundreds of restarts are everything
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
TrackMania United Forever
Time trials against ghosts - no one adapts to you
Grand Theft Auto V leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
TrackMania United Forever
Deterministic physics with literally nothing hidden
TrackMania United Forever leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
TrackMania United Forever
Finding the theoretical fastest line is the meta-game
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.