head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, and it is a single point.

Optimization · Practiced Mastery
The original loot-driven co-op shooter set on the bandit world of Pandora.

Optimization · Practiced Mastery
A S.T.A.L.K.E.R. prequel adding a dynamic faction war across the Zone.
Borderlands leans Optimization (Practiced Mastery); S.T.A.L.K.E.R.: Clear Sky leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Borderlands
Standard FPS aim; numbers matter more than precision
S.T.A.L.K.E.R.: Clear Sky
Punishing gunfights and anomaly navigation
S.T.A.L.K.E.R.: Clear Sky leans a little harder on reads, though the gap is small.
Borderlands
Bandits and skags are scripted
S.T.A.L.K.E.R.: Clear Sky
Faction-war AI is systemic, not adaptive to you
S.T.A.L.K.E.R.: Clear Sky leans a little harder on variance, though the gap is small.
Borderlands
Procedural loot drops gate your power
S.T.A.L.K.E.R.: Clear Sky
Dynamic faction war and emission RNG hide outcomes
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Borderlands
Builds, gear math, and farming routes dominate
S.T.A.L.K.E.R.: Clear Sky
Upgrade economy and territory routing 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.