head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Microsoft Flight Simulator (2020) demands more optimization.

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

Execution · Practiced Mastery
Global flight sim built on real weather and procedures.
Grand Theft Auto V leans Execution (Practiced Mastery); Microsoft Flight Simulator (2020) leans Execution (Practiced Mastery).
Microsoft Flight Simulator (2020) leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Microsoft Flight Simulator (2020)
Procedure execution, instrument flying skill
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Microsoft Flight Simulator (2020)
No adaptive opponents, ATC scripted
Microsoft Flight Simulator (2020) leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Microsoft Flight Simulator (2020)
Real weather and live traffic
Microsoft Flight Simulator (2020) leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Microsoft Flight Simulator (2020)
Flight planning, systems mastery, routing
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.