head to head
Neither is simply harder - they are hard in different places. Call of Duty: Modern Warfare 2 Campaign Remastered demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
Aim and spawn memorisation through a fully scripted cinematic campaign.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Call of Duty: Modern Warfare 2 Campaign Remastered leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Call of Duty: Modern Warfare 2 Campaign Remastered leans clearly harder on execution.
Call of Duty: Modern Warfare 2 Campaign Remastered
Aim and reaction carry every scripted firefight
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Duty: Modern Warfare 2 Campaign Remastered
Enemies spawn and behave on fixed scripts
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Grand Theft Auto: San Andreas leans a little harder on variance, though the gap is small.
Call of Duty: Modern Warfare 2 Campaign Remastered
Set-piece campaign with almost nothing random
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Call of Duty: Modern Warfare 2 Campaign Remastered
Learning spawns and cover lines trivialises veteran difficulty
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
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.
full breakdown
Call of Duty: Modern Warfare 2 Campaign Remastered →
full breakdown
Grand Theft Auto: San Andreas →