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
Frantic tag-team fighter crossing Capcom's mutants with World Warriors.
Street Fighter X Tekken leans Execution (Grand Arena); X-Men vs. Street Fighter leans Execution (Grand Arena).
X-Men vs. Street Fighter leans a little harder on execution, though the gap is small.
Street Fighter X Tekken
Tag combos and juggle timing demand practiced hands
X-Men vs. Street Fighter
Infinite combos and tag execution demand serious hands.
X-Men vs. Street Fighter leans a little harder on reads, though the gap is small.
Street Fighter X Tekken
Human mixups and tag baits require adaptation
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.
Street Fighter X Tekken
Deterministic aside from negligible elements
X-Men vs. Street Fighter
Fully visible game state.
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
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.