If I have interest in both aerospace and computer science. Then in which field among these will be a better option for placement from IIT's

As you are interested in two domains of engineering and also thinking about placements and also package provided by the company then you may choose computer science because in IIT's many of the software companies will show interest to take more number of people and you will have more opportunities t

aerospace and cse both gives you better placement . you have interest in both but how you see yourself in a mirror as an aeronautical engineer working in NASA or as an software engineer working in google.