Ideally Arusha, but JRO would be ok. Don't care if it leaves from Wilson or NBO. Who offers the best prices?

Between NBO and JRO there is only one carrier, Precision Air (can be booked as Kenya Airways codeshare) and the one way fare is around $200.

Air Kenya and Safarilink fly from Wilson to JRO. Both are about the same cost and slightly less than the Precision Air NBO-JRO flight, but if you're arriving in NBO, you'll have to factor in taxi fare to Wilson and possibly a transit visa (if you were otherwise not intending to spend time in Kenya), so the total cost may not be less.

Thanks Patty. I would be coming from a hotel in Nairobi so the either Kenya would involve a transfer/taxi fare.

Do all flights have to go thru JRO (i.e. you cannot fly non-stop Nairobi to Arusha)?

I think so. I may be wrong but I don't think there are immigration facilities at Arusha.

No flight from Kenya (Wilson or NBO's JKIA) goes to Arusha. Arusha is a domestic airport. Between Kenya/Tanzania - two countries you have to transit via an Int'l point of entry. Arusha is not.

The only deviation is the mid-day flight from Arusha/Wilson as Wilson has Immigration facilities available for this flight. But not in reverse as the mid-day flight from Wilson, goes into JRO (not Arusha).

Earlier this year, I was able to get the Precision Air flight (NBO - JRO) on Cheaptickets dot com for $170. That was less than the $200 that our contact in Nairobi was quoting.

$170 vs $200 ... that $30 difference buys a lot of peace of mind, though...

I'm not sure what you mean, NoFlyZone. We had paper tickets in hand and reconfirmed with Precision Air in Nairobi...simple, and saved $90 for three of us.

How bad is the drive from Nairobi to Arusha?

We took the bus from Nairobi to Arusha when were there in late June. We took the morning bus, 8am, although turned up at 8.30am. It took about 5.5 hours and I think was about $25 per person.
The roads are pretty bumpy and you have one rest stop for about 20 minutes. Passport control between Naiobi and Tanzania takes about 20 minutes on the Tanzania side, if there are quite a few people passing through , but only about five minutes on the Kenyan side. We snoozed on and off during the drive as we had been on the overnight flight into Nairobi.
We didn't have any issues with the journey.

I agree with Hamishbear about the description of the trip. It is OK - the road IS bumpy and our particular driver went pretty fast, but it is doable. So saying, if I am every lucky enough to find myself having to do this trip again, I would try and fly. Once really was enough!!