Both are excellent; preference depends on what you value.
Pros: Revolutionary War setting (Boston, New York, Frontier), naval missions, hunting, dynamic seasons, new combat (tomahawk, dual-wield), Connor's personal journey.
Cons: Slow start (3+ hrs tutorial), occasional bugs, Desmond arc divisive, frontier traversal tedious.
Pros: Rome fully explorable, refined combat (kill streaks, executions), Brotherhood system (recruit/train assassins), best-in-series Ezio story, VR training, multiplayer (unique, tense).
Cons: Single city (vs. AC3's variety), less innovation, side missions repetitive.
Most fans rank Brotherhood slightly higher; AC III's Remastered (2019) fixes many technical issues.