1) Which one is better and why?
2) What is the price of plot (25x50 and 25x40) in these area?
Depends on what you want. G-13 is a developed sector and has an excellent location so prices are already high but it is a good sector for building a house and living purpose. G-14 is still undeveloped so prices are a bit low. It is really good for investment but no one knows for sure when it will get developed completely.
Dear Bilal and Zohaib G-14/4 is also a developed sector both are good sectors but we can rank G-13 as a better one due to its location plus its more populated and another imp point that all of G-13 sub sectors are free of encroachment. Plot price in G-14/4 size 25 x 50 is around 4 to 4.5 million roughly.