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

Execution · Grand Arena
Gunfight execution and human prediction under randomised loot and storms.

Optimization · Grand Arena
Blizzard's hero-centric RTS where build-order theory meets surgical unit micro against humans.
Ring of Elysium leans Execution (Grand Arena); Warcraft III: Reign of Chaos leans Optimization (Grand Arena).
Ring of Elysium leans a little harder on execution, though the gap is small.
Ring of Elysium
Gunfights are won by aim and movement first
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.
Ring of Elysium
Outguessing human rotations and pushes is constant
Warcraft III: Reign of Chaos
Scouting and out-guessing a human opponent's tech
Ring of Elysium leans clearly harder on variance.
Ring of Elysium
Loot spawns and storm path randomise every match
Warcraft III: Reign of Chaos
Creep routes and item drops add randomness
Warcraft III: Reign of Chaos leans clearly harder on optimization.
Ring of Elysium
Drop routes and rescue-flight positioning reward planning
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.