You don't really have choice. Officially everything is centralised in the visitor's center. The only thing you can do there is chose what kind of tour you want to do and pay the regular price.
Worse, you don't even chose your driver (I wouldn't call him a guide!), he is assigned to you!
If you are unlucky he will barely speak English. In my case I had the chance to be with an arab friend who could do the translation...
Don't expect too much explanations from this guy neither, his job is to drive you around and that's about it!
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At Least Do This: Be ready and check before the different things to be seen and what you can expect in any given tour.
At least you could maybe negotiate to include some more spots in the regular tour and you are sure you don't get fooled by the driver.
During the different halts, take your time and don't be afraid to walk around if you feel this is going too fast.
Our 4hours tours was indeed a 3h30 tour!
Even worse before the end, our driver passed by his own house, showed us his kids, expecting some kind of tip!
Alternative: To bypass the visitors' center, there are some local guides you can contact before arriving, either by phone or via the web!
Make some search and try to find one of those, in any way, they can't be worse...