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

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

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Assassin's Creed Brotherhood leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Grand Theft Auto V leans a little harder on execution, though the gap is small.
Assassin's Creed Brotherhood
Kill-streak combat is stylish but easy
Grand Theft Auto V
Driving and shooting demand decent inputs.
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
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Assassin's Creed Brotherhood
Fixed campaign content
Grand Theft Auto V
Some mission and lobby randomness.
Assassin's Creed Brotherhood leans a little harder on optimization, though the gap is small.
Assassin's Creed Brotherhood
Recruit management and shop renovation economy
Grand Theft Auto V
Heists, money loops, builds learnable.
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.