head to head
Neither is simply harder - they are hard in different places. Command & Conquer: Renegade demands more reads; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
FPS aim welded to a team economy of base-destruction priorities.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Command & Conquer: Renegade leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Command & Conquer: Renegade leans clearly harder on execution.
Command & Conquer: Renegade
FPS aim in infantry and vehicle fights carries the game
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Command & Conquer: Renegade leans clearly harder on reads.
Command & Conquer: Renegade
Multiplayer base assaults involve predicting human team pushes
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.
Command & Conquer: Renegade
Maps and economy rules are fully known
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Command & Conquer: Renegade
Credit economy and building-target priority decide C&C-mode matches
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.