While the Aerospace department of IIT Kharagpur is indeed well developed, IISc is still the best option for Aerospace in India. It has multiple collaborations with reputed Aircraft Manufacturing companies like Boeing and Airbus offering you the necessary exposure that might not be possible at IIT KGP. You can get an insight into the working of these companies and learn things quickly.
You can even apply to the foreign branches of these companies and get a good job. IISc is well known for its enthusiastic research-oriented approach and learned faculty that will help you grow as a person.
Being a knowledgeable person does necessarily mean you have to secure full marks. Knowing the right set of skills and implementing them is a must to be one of the top students.
That being said, to achieve great things you cannot expect your journey to be a cakewalk. The workload at IISc is indeed much more than IIT Kharagpur. You will face continuous challenges as you stay in the dynamic research environment which will eventually become a thrilling experience. Going for IISc will definitely guarantee you a good career ahead.