head to head
Neither is simply harder - they are hard in different places. Command & Conquer 3: Tiberium Wars demands more optimization; Overwatch demands more execution.

Optimization · Grand Arena
Fast tiberium-economy RTS with a once-thriving competitive 1v1 scene.

Execution · Grand Arena
Team hero shooter blending FPS mechanics with ability economy mind games.
Command & Conquer 3: Tiberium Wars leans Optimization (Grand Arena); Overwatch leans Execution (Grand Arena).
Overwatch leans clearly harder on execution.
Command & Conquer 3: Tiberium Wars
Fast micro on squads and harvesters
Overwatch
Hero aim, movement, and ability timing
Overwatch leans a little harder on reads, though the gap is small.
Command & Conquer 3: Tiberium Wars
Ranked 1v1 rewards reading enemy tech paths
Overwatch
Ult tracking, flanks, and baiting human teams
Command & Conquer 3: Tiberium Wars leans a little harder on variance, though the gap is small.
Command & Conquer 3: Tiberium Wars
Fog hides tech switches until scouted
Overwatch
Minor spread and matchmaking noise
Command & Conquer 3: Tiberium Wars leans clearly harder on optimization.
Command & Conquer 3: Tiberium Wars
Tight build orders and tiberium control
Overwatch
Team comps, ult economy, and positioning theory
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.