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
Methodical dungeon mapping and puzzle-solving with token real-time combat.
Fallout 3 leans Optimization (Pure Optimization); Stonekeep 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
Stonekeep
Real-time mouse swings need only basic timing
Fallout 3 leans a little harder on reads, though the gap is small.
Fallout 3
Scripted enemies
Stonekeep
Dungeon denizens follow fixed scripts
Stonekeep leans a little harder on variance, though the gap is small.
Fallout 3
Loot rolls, SPECIAL checks
Stonekeep
Unmapped corridors and secrets are scripted-but-unknown
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
Stonekeep
Mapping, rune magic and inventory puzzles are the real test
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.