head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Metal Gear Solid 2: Substance demands more optimization.

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

Optimization · Practiced Mastery
A stealth-action game where planning routes past scripted guards is the deep skill.
Grand Theft Auto V leans Execution (Practiced Mastery); Metal Gear Solid 2: Substance leans Optimization (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Metal Gear Solid 2: Substance
stealth timing and aim matter
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Metal Gear Solid 2: Substance
guards are scripted, no adaptation
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Metal Gear Solid 2: Substance
minor patrol randomness
Metal Gear Solid 2: Substance leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Metal Gear Solid 2: Substance
stealth routes and gadgets 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.