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

Execution · Practiced Mastery
A lean souls-like carried by hardened-parry timing and shell selection.

Execution · Practiced Mastery
Combo execution and line routing through fixed skateparks for maximum score.
Mortal Shell leans Execution (Practiced Mastery); Tony Hawk's Proving Ground leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Hardened parry timing demands inputs
Tony Hawk's Proving Ground
Perfect inputs chain manuals, grinds and Nail-the-Trick combos indefinitely
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Scripted bosses
Tony Hawk's Proving Ground
Multiplayer score battles barely involve predicting opponents
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Minor item randomness
Tony Hawk's Proving Ground
Skateparks are fixed and fully learnable
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Mortal Shell
Shell choice and upgrade routing matter
Tony Hawk's Proving Ground
Routing the highest-scoring combo line through terrain is the meta
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.