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
Optimizing pair-up chains and offspring builds to steamroll deterministically scripted maps.

Optimization · Pure Optimization
A merge-puzzle auto-battler about composing strong squads to retake castles.
Fire Emblem Fates leans Optimization (Pure Optimization); Merge & Blade 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.
Fire Emblem Fates
Turn-based menu play; no hand-speed demands
Merge & Blade
Placing and merging units, no twitch input
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Fire Emblem Fates
AI follows scripted attack priorities, not player modeling
Merge & Blade
Monster armies scripted, not adaptive opponents
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Fire Emblem Fates
Dual-strike/guard procs and hit rates inject meaningful RNG
Merge & Blade
Unit draws from merging inject roguelike randomness
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Fire Emblem Fates
Offspring seals, asset/flaw selection, and supports are deeply planned
Merge & Blade
Building optimal merged squads for auto-battles is the game
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.