head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is reads, 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 tag-team 3D fighter layering team optimization onto Tekken's execution and reads core.
Street Fighter X Tekken leans Execution (Grand Arena); Tekken Tag Tournament 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 X Tekken
Tag combos and juggle timing demand practiced hands
Tekken Tag Tournament
Tag combos and netsu juggles demand precise inputs
Tekken Tag Tournament leans a little harder on reads, though the gap is small.
Street Fighter X Tekken
Human mixups and tag baits require adaptation
Tekken Tag Tournament
Tag timing adds another layer of human mind-games
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
Tekken Tag Tournament
Open information; nothing hidden between rounds
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
Tekken Tag Tournament
Team synergy and tag combo routing are optimizable systems
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.