Doing B.Tech in Aerospace engineering and  M.Tech in the same has their own separate benefits. If you don't want to study for longer duration and want a well paid job in short time span of 4 years then B.tech is for you. You will have good career opportunities in ISRO and NASA.
But if you want to enhance your skills and want to have advance training in science and technology and you can pursue M.Tech . This course is a 2year duration and is divided into 4 semester. After doing this course you will have knowledge in fields like space engineering, flight control and stimulation, advanced Aerodynamics.

Hope it helps !