head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Impossible Mission demands more variance.

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

Execution · Practiced Mastery
C64 classic about somersaulting past robots to assemble a password.
Grand Theft Auto V leans Execution (Practiced Mastery); Impossible Mission leans Execution (Practiced Mastery).
Impossible Mission leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Impossible Mission
Pixel-precise flips past patrolling robots
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Impossible Mission
Robots patrol on fixed behaviours
Impossible Mission leans clearly harder on variance.
Grand Theft Auto V
Some mission and lobby randomness.
Impossible Mission
Puzzle pieces randomized each playthrough
Impossible Mission leans a little harder on optimization, though the gap is small.
Grand Theft Auto V
Heists, money loops, builds learnable.
Impossible Mission
Search routing under the time limit
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.