head to head
Neither is simply harder - they are hard in different places. StarCraft demands more optimization; X-Men vs. Street Fighter demands more execution.

Optimization · Grand Arena
A real-time strategy classic where perfected build orders and mechanical speed decide most matches.

Execution · Grand Arena
Frantic tag-team fighter crossing Capcom's mutants with World Warriors.
StarCraft leans Optimization (Grand Arena); X-Men vs. Street Fighter leans Execution (Grand Arena).
X-Men vs. Street Fighter leans clearly harder on execution.
StarCraft
APM and micro control ceiling is huge
X-Men vs. Street Fighter
Infinite combos and tag execution demand serious hands.
X-Men vs. Street Fighter leans clearly harder on reads.
StarCraft
scouting and predicting human opponent builds
X-Men vs. Street Fighter
Human opponents adapt mixups every round.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
StarCraft
minimal hidden info beyond fog of war
X-Men vs. Street Fighter
Fully visible game state.
StarCraft leans clearly harder on optimization.
StarCraft
build orders and economy are near-solvable
X-Men vs. Street Fighter
Team synergy and matchup theory reward lab time.
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.