head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is execution, and it is a single point.

Execution · Grand Arena
A tag-team crossover fighter pitting Street Fighter's cast against Tekken's under Capcom rules.

Execution · Grand Arena
A hyper-fast tag fighting game defined by three-character teams and touch-of-death combos.
Street Fighter X Tekken leans Execution (Grand Arena); Ultimate Marvel vs. Capcom 3 leans Execution (Grand Arena).
Ultimate Marvel vs. Capcom 3 leans a little harder on execution, though the gap is small.
Street Fighter X Tekken
Tag combos and juggle timing demand practiced hands
Ultimate Marvel vs. Capcom 3
Long touch-of-death combos demand elite hands
Ultimate Marvel vs. Capcom 3 leans a little harder on reads, though the gap is small.
Street Fighter X Tekken
Human mixups and tag baits require adaptation
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 X Tekken
Deterministic aside from negligible elements
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 X Tekken
Gem loadouts and team synergy add a builder layer
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.