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

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

Optimization · Practiced Mastery
A real-time RPG carried by ascension-grid routing and magic crafting.
Fallout 3 leans Optimization (Pure Optimization); Final Fantasy XV leans Optimization (Practiced Mastery).
Final Fantasy XV leans a little harder on execution, though the gap is small.
Fallout 3
VATS-leaning combat, low aim demand
Final Fantasy XV
Real-time combat, modest input demand
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fallout 3
Scripted enemies
Final Fantasy XV
Scripted enemies
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
Final Fantasy XV
Loot and ascension variance
Fallout 3 leans a little harder on optimization, though the gap is small.
Fallout 3
Perk and build planning carries the run
Final Fantasy XV
Ascension grid, magic crafting, party 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.