We used Sunni Safaries in Arusha and had a very good experience with them. Would recommend them to anybody!

When planning the trip, we put together an itinerary of what we would like to do and sent it to about a dozen or so tour companies in Arusha. Most of them responded and they pretty much came up with the same tours and same lodges. However, the price differences were sometimes 100% or more!
I have read other reports were Sunni was much more than other outfits, so I guess it really depends who has the best rates at a given time. I would just send a request for a quote to a bunch of companies and see what they come back with. (Used the recommended list of companies in Lonely Planet, Frommers, and a two or three that were recommended on trip reports I found.)