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

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

Execution · Competitive PvP
The fourth Soulcalibur adds armor-breaking Soul Gauge pressure and Critical Finishes to its weapon dueling.
Mortal Kombat: Deception leans Execution (Competitive PvP); Soulcalibur IV leans Execution (Competitive PvP).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Kombat: Deception
Style strings plus stage-hazard spacing demand control
Soulcalibur IV
Step-guard and just-guard timing keep a real physical ceiling.
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
Soulcalibur IV
Critical Finish threats add layers to adaptive mixup guessing.
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
Soulcalibur IV
Soul Gauge is visible; no hidden info.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Kombat: Deception
Combo routes and hazard positioning are lab work
Soulcalibur IV
Guard-crush pressure routes and frame data are studyable.
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.