head to head
Neither is simply harder - they are hard in different places. Halo Wars 2 demands more variance; StarCraft demands more optimization.

Execution · Grand Arena
Console-friendly RTS where leader powers, base management, and Blitz card battles test both micro and opponent reading.

Optimization · Grand Arena
A real-time strategy classic where perfected build orders and mechanical speed decide most matches.
Halo Wars 2 leans Execution (Grand Arena); StarCraft leans Optimization (Grand Arena).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Halo Wars 2
Leader power activation and Blitz card plays need precise timing
StarCraft
APM and micro control ceiling is huge
Halo Wars 2 leans a little harder on reads, though the gap is small.
Halo Wars 2
Opponent leader choice and Blitz deck strategy must be predicted
StarCraft
scouting and predicting human opponent builds
Halo Wars 2 leans clearly harder on variance.
Halo Wars 2
Blitz card draw and resource crate spawns add moderate uncertainty
StarCraft
minimal hidden info beyond fog of war
StarCraft leans clearly harder on optimization.
Halo Wars 2
Leader power cooldown sequencing and base slot assignment are learnable
StarCraft
build orders and economy are near-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.