The Serengeti and Ngorongoro Crater are Tanzania's two most famous safari destinations. They're 30 minutes apart but feel like different planets.
The Serengeti — vast and unbroken
14,750 km² of open plains, riverine forest and granite kopjes. The home of the Great Migration. Wildlife is spread across enormous distances, so a vehicle and a good guide matter enormously.
Ngorongoro Crater — compact and dense
260 km² of crater floor. The highest density of large mammals in Africa. Black rhino, lion prides, elephant, hippo and flamingo all within minutes of each other.
When to pick the Serengeti
If the migration is your priority. If you want vast, cinematic landscapes. If you have 5+ days.
When to pick Ngorongoro
If you have only 3-4 days. If you want the best Big-Five odds in one day. If you want to combine wildlife with Maasai culture.
The honest answer
Most northern-circuit itineraries include both, because they complement each other. The Serengeti for the depth and migration, Ngorongoro for the density and the rhino.