The Masai Mara and the Serengeti are ecologically one continuous landscape, split by a national border. If you're choosing based purely on 'which country has better wildlife', the honest answer is that the underlying ecosystem is shared - the differences are more about logistics, cost and trip structure.
Kenya's parks (Mara, Nakuru, Amboseli) tend to be smaller and closer together, which suits shorter trips with less driving between stops. Tanzania's northern circuit parks (Serengeti, Ngorongoro, Tarangire) are more spread out, generally suiting slightly longer itineraries to make the travel time worthwhile.
Tanzania's conservation and park fees are, park for park, generally higher than Kenya's equivalents, which tends to push overall trip costs up somewhat for a comparable standard of accommodation. Kenya's shorter distances between parks can also mean less time (and cost) spent on transfers.
Kenya is strong for shorter trips, the Mara's concentrated game viewing, and combining a safari with a Kenyan coast beach extension (Diani). Tanzania is strong for the Ngorongoro Crater's reliable big-five game drives, the sheer scale of the Serengeti, and combining a safari with Zanzibar.
Because the Mara and Serengeti are connected, it's possible to do an overland safari that starts in one country and ends in the other, crossing the border partway through - see our 8 Day Kenya & Tanzania Safari for an example of how that works in practice.
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