head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Practiced Mastery
Steady aim against scripted alien swarms with an upgrade economy underneath.

Execution · Practiced Mastery
A cartoonish horde shooter sequel of planet-hopping monster massacres.
Alien Breed: Impact leans Execution (Practiced Mastery); Serious Sam 2 leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Alien Breed: Impact
Kiting alien swarms top-down demands steady aim and movement
Serious Sam 2
Sustained horde-clearing aim and strafing
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Alien Breed: Impact
Aliens swarm on scripts, never modelling you
Serious Sam 2
Enemy waves are entirely scripted
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Alien Breed: Impact
Ambush triggers are scripted-but-unknown first time through
Serious Sam 2
Cartoon set-pieces surprise once
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Alien Breed: Impact
Weapon upgrades and ammo economy reward planning
Serious Sam 2
Target priority and arena routes are learnable
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.