If you wish to go for the MS in Aerospace Engineering, then apart from the BTech in Aerospace Engineering, you can go for the Mechanical Engineering. Mechanical Engineering is a broader field and the curriculum of Mechanical Engineering can make your life ahead easier and as an Aerospace Engineer, your background of Mechanical Engineering will be of great value. So if you wish to go for any other fields, then I would recommend you to go for the course of  Mechanical Engineering.

For more information regarding Aerospace Engineering consider the link below:

https://www.careers360.com/courses/aerospace-engineering-course

For more information regarding Mechanical Engineering consider the link provided below:

https://www.careers360.com/courses/mechanical-engineering-course

I hope this helps. Wish you a great day!