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 · Pure Optimization
A VATS-heavy wasteland RPG won by perk and build optimization.

Optimization · Pure Optimization
Preparation-driven RPG combat where potions, builds and monster knowledge beat reflexes.
Fallout 3 leans Optimization (Pure Optimization); The Witcher: Enhanced Edition Director's Cut 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.
Fallout 3
VATS-leaning combat, low aim demand
The Witcher: Enhanced Edition Director's Cut
Rhythm-click combat is timing-light and forgiving
Fallout 3 leans a little harder on reads, though the gap is small.
Fallout 3
Scripted enemies
The Witcher: Enhanced Edition Director's Cut
Enemies follow fixed behaviours, nothing adapts to you
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 3
Loot rolls, SPECIAL checks
The Witcher: Enhanced Edition Director's Cut
Quest outcomes are scripted-but-unknown on a first playthrough
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 3
Perk and build planning carries the run
The Witcher: Enhanced Edition Director's Cut
Alchemy prep, signs and talent builds decide every fight
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.