Hi,
I really appreciate your interest in starting a startup  in aviation related industry.  But As far india, more over Aerospace and aeronautical engineering are same.
But basically Aeronautics is all about dealing with aircrafts which are operational within the atmosphere like aeroplanes, helicopters etc.
Astronautics is all about dealing with aircrafts which are operational beyond the atmosphere (also called spacecraft) like space shuttles etc.
Hence, Aerospace engineering deals with both aeronautics and astronautics.
Master in aeronautical  specialisation ,will make career in aviation sector. 
Master in aerospace or space technology will give enough knowledge and make you capable in establishing a space related startup
After btech level , you can choose aerospace or aeronautical,  it doesnot matter a lot