The Serengeti and the Maasai Mara are part of the same ecosystem, connected by the Mara River, and the wildebeest migration moves between both — so the choice between them usually comes down to timing, scale and travel style rather than one being simply “better”.
Scale
The Serengeti is considerably larger than the Mara, with more varied terrain across its different regions, from the short-grass southern plains to the wooded north near the Kenyan border.
Timing
If your dates are fixed around river crossings, Kenya’s Mara is the stronger choice from roughly July–October; if you want calving season and the predator action around newborn calves, Tanzania’s southern Serengeti from January–March is the better fit.
Can you do both?
Yes — some itineraries cross the border within the same trip, though this adds cost and logistics. For most travellers, choosing one side and building the rest of the itinerary around it (Ngorongoro Crater alongside the Serengeti, or the Kenyan coast alongside the Mara) makes for a simpler, still excellent trip.