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
Instanced online RPG where deck-like eight-skill builds decide battles before they begin.

Optimization · Pure Optimization
RTS where you splice animals into custom armies before battle begins.
Guild Wars leans Optimization (Pure Optimization); Impossible Creatures 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.
Guild Wars
Interrupt timing and kiting in skirmishes
Impossible Creatures
Standard RTS micro and army control
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Guild Wars
GvG and arena metagame against adapting humans
Impossible Creatures
Human opponents counter your creature picks
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Guild Wars
Modest drop and spawn randomness
Impossible Creatures
Scouting hidden enemy combinations
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Guild Wars
Eight-skill build construction is the game's soul
Impossible Creatures
Creature-combining army design is deeply solvable
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.