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

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

Optimization · Practiced Mastery
A Revolutionary War entry mixing frontier stealth, naval battles, and homestead building.
Assassin's Creed Brotherhood leans Optimization (Practiced Mastery); Assassin's Creed III 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
Assassin's Creed III
Frontier traversal and counter combat stay forgiving
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Assassin's Creed Brotherhood
Hide-and-hunt multiplayer ran on human mind-games
Assassin's Creed III
Multiplayer mode involved human deception
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Assassin's Creed Brotherhood
Fixed campaign content
Assassin's Creed III
Fixed story missions
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
Assassin's Creed III
Homestead economy and naval upgrades
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.