It all really comes down to cost and time. The main land border crossings between Kenya and Tanzania are quick and simple to cross with good roads leading to them and regular public transport. Visas are available on the borders and there are no hassles in crossing.
But, of course flying is faster (although once you factor in arriving at the airport two hours in advance then you're only likely saving a couple of hours over driving if talking about the main Nairobi-Arusha crossing at Namanga).
So, it’s really up to personal preference.
