head to head
Neither is simply harder - they are hard in different places. ARC Raiders demands more variance; Warcraft III: Reign of Chaos demands more optimization.

Execution · Grand Arena
An extraction shooter where scavengers dodge killer machines and rival squads for loot.

Optimization · Grand Arena
Blizzard's hero-centric RTS where build-order theory meets surgical unit micro against humans.
ARC Raiders leans Execution (Grand Arena); Warcraft III: Reign of Chaos 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.
ARC Raiders
Gunplay and movement under machine fire
Warcraft III: Reign of Chaos
Micro of heroes and units decides fights
Warcraft III: Reign of Chaos leans a little harder on reads, though the gap is small.
ARC Raiders
Rival squads ambush, bait, and adapt
Warcraft III: Reign of Chaos
Scouting and out-guessing a human opponent's tech
ARC Raiders leans clearly harder on variance.
ARC Raiders
Loot spawns and unknown player positions
Warcraft III: Reign of Chaos
Creep routes and item drops add randomness
Warcraft III: Reign of Chaos leans clearly harder on optimization.
ARC Raiders
Loadout economy and map knowledge compound
Warcraft III: Reign of Chaos
Build orders, creeping routes, and matchup theory run deep
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.