head to head
Neither is simply harder - they are hard in different places. Alien Breed 2: Assault demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
Kiting and shooting scripted alien swarms with modest resource management underneath.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Alien Breed 2: Assault leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Alien Breed 2: Assault leans clearly harder on execution.
Alien Breed 2: Assault
Twin-stick aim and kiting under swarm pressure decide survival
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.
Alien Breed 2: Assault
Alien hordes path predictably with no adaptation
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Alien Breed 2: Assault
Ambush placements are scripted-but-unknown on first run
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Alien Breed 2: Assault
Ammo economy and route knowledge smooth repeat playthroughs
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.