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 · Competitive PvP
The Deadly Alliance follow-up adding combo breakers, death traps, and online play.

Execution · Grand Arena
A tag-team crossover fighter pitting Street Fighter's cast against Tekken's under Capcom rules.
Mortal Kombat: Deception leans Execution (Competitive PvP); Street Fighter X Tekken leans Execution (Grand Arena).
Street Fighter X Tekken leans a little harder on execution, though the gap is small.
Mortal Kombat: Deception
Style strings plus stage-hazard spacing demand control
Street Fighter X Tekken
Tag combos and juggle timing demand practiced hands
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Kombat: Deception
Human opponents bait death traps and breakers
Street Fighter X Tekken
Human mixups and tag baits require adaptation
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Kombat: Deception
Deterministic outside known stage hazards
Street Fighter X Tekken
Deterministic aside from negligible elements
Street Fighter X Tekken leans a little harder on optimization, though the gap is small.
Mortal Kombat: Deception
Combo routes and hazard positioning are lab work
Street Fighter X Tekken
Gem loadouts and team synergy add a builder layer
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.