head to head
Neither is simply harder - they are hard in different places. Command & Conquer: Red Alert 3 demands more variance; StarCraft demands more optimization.

Reads · Grand Arena
Three-faction Cold-War RTS with naval combat where faction asymmetry and multi-front pressure are the core tests.

Optimization · Grand Arena
A real-time strategy classic where perfected build orders and mechanical speed decide most matches.
Command & Conquer: Red Alert 3 leans Reads (Grand Arena); StarCraft leans Optimization (Grand Arena).
StarCraft leans a little harder on execution, though the gap is small.
Command & Conquer: Red Alert 3
Naval and air micro alongside base-building taxes player bandwidth
StarCraft
APM and micro control ceiling is huge
Command & Conquer: Red Alert 3 leans a little harder on reads, though the gap is small.
Command & Conquer: Red Alert 3
Three-faction asymmetry means opponent choices must be constantly predicted
StarCraft
scouting and predicting human opponent builds
Command & Conquer: Red Alert 3 leans clearly harder on variance.
Command & Conquer: Red Alert 3
Ore spawning and naval choke variance add moderate uncertainty
StarCraft
minimal hidden info beyond fog of war
StarCraft leans clearly harder on optimization.
Command & Conquer: Red Alert 3
Production order and defensive placement are learnable but more flexible
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.