Start with time, not a country ranking
Ndurumo's Kenya catalogue includes short Nairobi-return routes and multi-stop circuits. Tanzania routes start and end in Arusha and stay within the northern circuit. Current combined routes need nine to twelve days because they join two national circuits rather than adding a simple day trip.
A traveller with three to five safari days can make a complete one-country choice: a Kenya route can focus on the Mara and Rift Valley lakes, while a Tanzania route can cover a compact northern-circuit sequence. Six to eight days gives one country more depth. If Kilimanjaro is also part of a separate trip, an Arusha-based Tanzania safari can fit naturally before or after the climb.
A first safari does not need every park
A focused one-country route can give you more time in fewer places. A combined route gives you a wider geographic story, but also adds a border crossing and more travel. Neither shape is automatically the better first safari.
For a first safari, choose the route whose main landscapes already excite you: Mara and Amboseli, Serengeti and Ngorongoro, or a deliberately paced combination. A repeat traveller may prefer the country that adds a new ecosystem to places already visited. In either case, count meaningful time in each destination rather than judging the trip by how many park names appear in the title.
Use migration timing as geography, not a guarantee
The annual movement spans the Serengeti and Maasai Mara and follows rainfall rather than a fixed timetable. Choose the likely part of the ecosystem for your month, then treat any river crossing as a possibility rather than a promise.
The practical answer
If you are still undecided, start with your dates and the number of safari days you can use. Then choose the two or three places you would be most disappointed to miss. Those facts usually narrow the current catalogue to one country or one realistic combined route.