head to head
Neither is simply harder - they are hard in different places. Command & Conquer: Tiberian Sun demands more variance; StarCraft demands more optimization.

Optimization · Grand Arena
Asymmetric sci-fi RTS where tiberium economy management and faction counter-play are the decisive layers.

Optimization · Grand Arena
A real-time strategy classic where perfected build orders and mechanical speed decide most matches.
Command & Conquer: Tiberian Sun leans Optimization (Grand Arena); StarCraft leans Optimization (Grand Arena).
StarCraft leans a little harder on execution, though the gap is small.
Command & Conquer: Tiberian Sun
Stealth unit deployment and artillery positioning need precise timing
StarCraft
APM and micro control ceiling is huge
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Command & Conquer: Tiberian Sun
NOD vs GDI asymmetry means opponent strategy must be predicted
StarCraft
scouting and predicting human opponent builds
Command & Conquer: Tiberian Sun leans clearly harder on variance.
Command & Conquer: Tiberian Sun
Tiberium field growth and map elevation create meaningful uncertainty
StarCraft
minimal hidden info beyond fog of war
StarCraft leans clearly harder on optimization.
Command & Conquer: Tiberian Sun
Harvester routing and unit counter choices are deeply solvable
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.