Just to add to the last post...
JK is just as bureaucratic and tedious to deal with. It all depends on what you or the pax are planning to do when you arrive. Refuel and leave again asap - use JK or local sightseeing - use Wilson.
Getting to the Mara should be easy enough. There's enough schedule airlines operating there - lots of good, bad and indifferent operators. Same for the charter operators - some good and proficient, some just learning, and others with just a shiny coat of paint.
The 'big' charter operators are Boskys, East African and in third place Yellow Wings. They all have Caravans and C206's - Bosky and East African both have twin turboprops as well for VIPs.
It is pretty busy with the migration out in the Mara, so hotel bookings might be more of an issue than a seat or four on a flight. Don't leave things to the last minute - rooms and seats are a priority at the moment.