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.

Optimization · Practiced Mastery
Ezio rebuilds the Assassin order in Rome, with a cult-classic mind-game multiplayer attached.

Optimization · Practiced Mastery
San Francisco hacker sandbox blending stealth, drones, and toy gadgets.
Assassin's Creed Brotherhood leans Optimization (Practiced Mastery); Watch Dogs 2 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.
Assassin's Creed Brotherhood
Kill-streak combat is stylish but easy
Watch Dogs 2
Shooting and driving moderate input demands.
Assassin's Creed Brotherhood leans a little harder on reads, though the gap is small.
Assassin's Creed Brotherhood
Hide-and-hunt multiplayer ran on human mind-games
Watch Dogs 2
Enemy AI scripted; some PvP invasions exist.
Watch Dogs 2 leans a little harder on variance, though the gap is small.
Assassin's Creed Brotherhood
Fixed campaign content
Watch Dogs 2
Hack outcomes and patrols vary mildly.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Assassin's Creed Brotherhood
Recruit management and shop renovation economy
Watch Dogs 2
Gadgets, hacks, mission approaches 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.