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
A cartoonish horde shooter sequel of planet-hopping monster massacres.

Execution · Practiced Mastery
Kiting scripted alien swarms with top-down aim and upgrade planning.
Serious Sam 2 leans Execution (Practiced Mastery); Shadowgrounds Survivor 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.
Serious Sam 2
Sustained horde-clearing aim and strafing
Shadowgrounds Survivor
Kiting alien swarms with top-down aim is the core demand
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Serious Sam 2
Enemy waves are entirely scripted
Shadowgrounds Survivor
Aliens charge on simple scripted aggression
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Serious Sam 2
Cartoon set-pieces surprise once
Shadowgrounds Survivor
Spawn waves vary slightly in survival mode
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Serious Sam 2
Target priority and arena routes are learnable
Shadowgrounds Survivor
Weapon upgrade order and positioning reward planning
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.