Dear Rizi,
You yourself are a very informed person so am sure you'll pick up the right choice. :)
Although your question is about E16 and B17 but I want to digress to zone 1 today.
It seems your purpose is investment and for that sake, believe me nothing is better than zone 1, CDA sectors.
G14/3 and G14/2 encroachment free areas are a prime time option these days, Allah knows the ultimate future but on analytical basis this has to be your choice.
In zone 2, there is a mushroom growth of societies and they will never stop coming so don't think you'll miss out some golden chances.
Short term lucky flooks can happen sometimes but u can't base your decisions on that.
Please keep in mind, no matter where in the world u are, DISTANCE and Location matters more than anything else. So even if the Margalla Avenue is built in next 5-7 yrs, your car will still need petrol to travel the distance. :)
The price increase has been pervasive in Islamabad with a speculation of government change and elections. Soon after the elections, if the same govt and allies come in power, all property prices will go down.
On the other hand, If, as speculated, a good govt with stronger majority comes in then of course you'll reap the fruits.
As a personal advice, don't take the bait of private societies, no matter which one is that, its very short lived.