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

Optimization · Pure Optimization
Stacking multi-class abilities and CT tricks to dominate tactically scripted encounters.

Optimization · Pure Optimization
A 3rd Edition ruleset combat puzzle where multi-classing and feat selection unlock dominant strategies.
Fell Seal: Arbiter's Mark leans Optimization (Pure Optimization); Icewind Dale II leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Fell Seal: Arbiter's Mark
Turn-based; no timing windows whatsoever
Icewind Dale II
Real-time-with-pause negates twitch requirements
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fell Seal: Arbiter's Mark
Enemy AI is scripted; fixed job archetypes behave predictably
Icewind Dale II
Scripted enemy routines offer nothing to read or predict adaptively
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Fell Seal: Arbiter's Mark
Random encounters and loot add mild variance; counter procs are minor
Icewind Dale II
3E feat combinations and hidden enemy resistances create unknown-information layers
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fell Seal: Arbiter's Mark
Multi-class ability transfers and CT manipulation make this near-FFT optimization depth
Icewind Dale II
3rd Edition feat optimization and multi-class architecture dominate success
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.