Both G-13 and D-12 are good.
G-13 for immediate living, relatively cheaper, closer to city, Kashmir highway, relatively lesser rental income compared to D12
D-12 for living after 2-3 yrs, expensive but will give higher rentals too, close to planned Margalla Avenue, beautiful location in front of Margalla Hills, will offer high end elite neighborhood.
In terms of price difference, D-12 will always be expensive than any of the new sectors.
Overall, current prices for both sectors are inflated if judged on value for money as there are question marks on provision of utilities like water, gas etc.
Very Slow pace of construction.
My conservative assumption would be that these sectors will mature/fully populate:
G-13 in 2-3 yrs
D-12 in 3-5 yrs