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

Optimization · Practiced Mastery
A charming kid-friendly RPG with timing prompts and light costume strategy.

Optimization · Practiced Mastery
A cinematic sci-fi side-scroller about a family fleeing alien invasion through quiet environmental puzzles.
Costume Quest leans Optimization (Practiced Mastery); Somerville 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.
Costume Quest
Timed button prompts are the only dexterity ask
Somerville
Light chase sequences need basic timing.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Costume Quest
Monster battles follow simple fixed scripts
Somerville
Alien threats are scripted set pieces.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Costume Quest
Quest content is fixed once doors are knocked
Somerville
Fixed authored world; minimal hidden state.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Costume Quest
Costume ability matchups and stamp choices are light strategy
Somerville
Environmental light-matter puzzles with intended solutions.
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.