I did AMS as the stopover on the way to Kilimanjaro airport. Keep in mind that Nairobi to Arusha by bus is a whole different experience than JRO into Arusha.

For connecting, I would recommend one of European cities. I'd prefer being stuck in Amsterdam than Addis if the flight is canceled or my luggage is lost during the connection.

You're also likely to avoid a visa/entry fee in the European stops when you leave the airport.