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 · Competitive PvP
High-speed tag-team execution and human prediction in gleeful early-Versus chaos.

Execution · Grand Arena
A variation-based fighter where character specialisation choices and read-and-punish execution drive competitive outcomes.
Marvel vs. Capcom: Clash of Super Heroes leans Execution (Competitive PvP); Mortal Kombat X 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.
Marvel vs. Capcom: Clash of Super Heroes
Chain combos, assists and infinites demand fast precise hands
Mortal Kombat X
Variation-specific combo routes demand precise practice
Mortal Kombat X leans a little harder on reads, though the gap is small.
Marvel vs. Capcom: Clash of Super Heroes
Baiting a human's assist call or super is the duel
Mortal Kombat X
Reading opponent's variation choice and tendency shifts the matchup
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Marvel vs. Capcom: Clash of Super Heroes
Hidden characters aside, everything is public
Mortal Kombat X
No RNG; transparent match flow
Mortal Kombat X leans a little harder on optimization, though the gap is small.
Marvel vs. Capcom: Clash of Super Heroes
Team composition and infinite-combo knowledge define the meta
Mortal Kombat X
Variation selection and optimal punish routes require systematic study
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.