The success of your trip depends on many factors.  Budget is a major consideration for most people, but the cheapest option is frequently not the best option.  You need to look at what it costs to get to Africa in the first place, figure out what your goals are, research the areas you want to see, and make a lot of enquiries to safari companies, stating your goals and budget.
 
The cheapest option is to wait until you get to Arusha and hop onto a safari on a space available basis.  That is also the riskiest for several reasons.  The safaris may be booked up at that point, and you would be stuck.  And, there is the question of reputation- many last-minute safaris are not reputable, and you will either be ripped off, or have a very inferior trip with a less-than-knowledgeable driver.
 
I can speak from experience that having a knowledgeable driver is paramount to having a good safari, unless you are so experienced that you don't need one.  We needed not only our driver's brains, but his eyes as well.  His ability to spot wildlife was above excellent, and he had encyclopedic knowledge of all the animals we came across.  He had reference books, and could look up additional information.  He also had knowledge of all the obscure tracks, and had very good intuition about where to look to find this animal or that animal (cheetahs and leopards, for instance).  And finally, he was East African Safari Land Roverconnected in with the other drivers, and they would exchange knowledge about where the animals were, and he knew the system well- what fee had to be paid for which permit and where to submit them.
 
We queried four safari companies that were listed in the Lonely Planet Guide as reputable- East African Safari & Touring Company, Roy's Safari, Safari Makers, and Sunny Safaris.  East African was the most expensive option, but also closely matched our goals and, if you will, our requirements.  Also, they offered a private safari, just us and the driver, so we got exactly what we wanted out of the trip.
 
Generally, the cheaper the safari, the more people there will be in your vehicle.  We saw some vans with 10 or more people in them.  If you just want to see wildlife, and have little control over what you see and how long you stay in one place, then perhaps a cheaper option is fine.  Being a photographer, I wanted complete control (as much as possible) to optimize around picture taking.
 
One company, Sunny Safaris, had given us the cheapest quote.  We saw Sunny Safari vehicles on while on our safari, and I can say that I'm very happy we did not choose them!  Their vans were packed full of people, and in one case, by the Hippo Pool in Ngorongoro Crater, the driver of the Sunny Safari vehicle would not shut his vehicle off.  The safari vehicles, now mind you, are diesel, and therefore are a bit noisy when idling.  This constant background noise made it difficult to videotape the hippos and get the sounds of them wallowing and snorting at each other.
 
East African Safari Land RoverHere are some things to expect with regard to the vehicles- they need to be turned off when you want to take pictures.  In general, the animals are not right next to the vehicle, and you need to use telephoto.  If the vehicle is idling, it is vibrating and that will ruin your pictures, not to mention your video.  And the sound will permeate your video, wiping out the sounds of Africa- something we found important to our total experience.
 
The Sunny Safaris driver's refusal to turn off his vehicle, was in my opinion, reprehensible.  I motioned to them to turn off the vehicle, and a tourist in the vehicle indicated that this driver would not do that for them.  So, their safari was compromised, and they had a lesser experience.
 
Peter, our driver and guidePeter (to the right), our driver and guide for East African Safaris, turned the vehicle off every time we stopped, so I could get a stable, vibration-free picture, not to mention, pick up the natural sounds sans vehicle noise.  Peter also complied with our requests to a tee.  He never complained, was ever-so-helpful, and in the end, we felt we made a good decision with East African Safari, and that our money was well spent.
 
In addition, pre-trip, Simon King, the Australian owner of East African Safari, was very helpful and patient with my sometimes endless questions.  He sent us a Word document that had many helpful tips on what to pack and what to expect.  Simon was great and helpful, and even though we never met him, he and his office staff completed the picture and made for a terrific safari.  I would use East Africa Safari again should we decide to return to the region.