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.

Execution · Practiced Mastery
Open-world western about an outlaw forced to hunt down his old gang.

Optimization · Practiced Mastery
An open-world action game where hacking the city's infrastructure is your main weapon.
Red Dead Redemption leans Execution (Practiced Mastery); Watch Dogs leans Optimization (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Red Dead Redemption
Dead Eye softens but gunplay still wants aim
Watch Dogs
Cover shooting and driving at modest difficulty
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Red Dead Redemption
Multiplayer skirmishes add light human prediction
Watch Dogs
Online invasions add a small human element
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Red Dead Redemption
Ambient events and spawns vary mildly
Watch Dogs
Guard placements are knowable on approach
Watch Dogs leans a little harder on optimization, though the gap is small.
Red Dead Redemption
Mission approaches and economy are learnable
Watch Dogs
Hacking chains and camera routes make stealth solvable
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.