head to head
Neither is harder. They score identically on all four axes, so the difference between them is in kind, not degree.

Execution · Grand Arena
A high-execution fighter where one-frame links and opponent modelling separate competitors.

Execution · Grand Arena
A hyper-fast tag fighting game defined by three-character teams and touch-of-death combos.
Street Fighter IV leans Execution (Grand Arena); Ultimate Marvel vs. Capcom 3 leans Execution (Grand Arena).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Street Fighter IV
One-frame links and FADC combos demand extreme motor precision
Ultimate Marvel vs. Capcom 3
Long touch-of-death combos demand elite hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Street Fighter IV
Reading opponent habits, wake-up patterns, and adaptations is the meta
Ultimate Marvel vs. Capcom 3
Mixups and assist baits versus adapting humans
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Street Fighter IV
Virtually no RNG; every outcome is skill-driven
Ultimate Marvel vs. Capcom 3
Almost fully deterministic
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Street Fighter IV
Frame data, punish windows, and matchup charts are deeply studied
Ultimate Marvel vs. Capcom 3
Team composition and matchup knowledge 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.