head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is optimization, and it is a single point.

Optimization · Pure Optimization
A cerebral puzzle-adventure that is essentially a logic puzzle to be fully solved.

Optimization · Pure Optimization
A puzzle island where learning each panel's hidden language solves everything.
Riven: The Sequel to Myst leans Optimization (Pure Optimization); The Witness leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Riven: The Sequel to Myst
no reflex or physical skill required
The Witness
No reflex demands; line drawing only.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Riven: The Sequel to Myst
no opponent present in the game
The Witness
No opponent; island is static.
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Riven: The Sequel to Myst
fully deterministic puzzle world
The Witness
Puzzles deterministic with no randomness.
The Witness leans a little harder on optimization, though the gap is small.
Riven: The Sequel to Myst
pure logic-puzzle solving, walkthrough-solvable
The Witness
Decoding the rule systems IS the whole game.
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.