The Serengeti and the Masai Mara are the same ecosystem split by a border, so the "which is better" question really comes down to trip length, budget, and timing rather than one being objectively superior.
Serengeti advantages
- Much larger area, so it feels less crowded even in peak season
- Hosts the migration's calving season (Feb–March) in the southern plains
- Easier to combine with Ngorongoro Crater and Zanzibar
Masai Mara advantages
- Shorter flights from Nairobi, less total travel time
- Dense concentration of predators year-round
- Best river-crossing drama, July–September
If your trip window is July–September and you want maximum drama for minimum travel time, the Mara wins. If you have a longer trip and want the migration's birth season plus Ngorongoro, the Serengeti wins.