The honest answer is that it depends entirely on your month, not on which park is 'better' in the abstract. December through July, the herds are almost always in Tanzania — calving in the southern Serengeti, then moving north through the central and western corridors — and the Masai Mara is comparatively quiet game country during this stretch, still excellent for resident wildlife but not migration viewing. August through October, the balance flips: the herds are typically split between the northern Serengeti and the Masai Mara, with the Mara often holding the denser concentration and the more famous crossings during peak weeks. The Serengeti is vastly larger (nearly 15,000 km² against the Mara's roughly 1,500 km², including its conservancies), which spreads sightings and vehicles out more and generally means a less crowded experience even in peak season. The Mara's conservancies — private land bordering the reserve with strict vehicle limits — are the best-kept secret for travelers who want Mara-level migration odds without Mara-level crowding. Our practical rule: book Tanzania for a December-July trip, Kenya (ideally with a conservancy stay) for August-October, and consider both only if your trip is long enough to genuinely justify the crossing between them.