Hey I hope you are doing good. Its all depend on your interest. As zoology, botany and chemistry are the main branches and have equal scope. I will  suggest you to go in that field in which your concepts are much clear so that you will be able to crack NET easily. As if you are not strong in that subject in which you take admission then most time of  ist year is consuming in clearing the small small concepts, if you take that subject where you have good command it also increase your self confidence and you will be able to perform good. As  zoology, chemistry, botany has itself very vast opportunities individually. So take time think deeply and then decide this.