Should I pursue B.Tech in mechanical engineering or in aerospace engineering in order to pursue M.Tech in aerospace?

B.Tech and M.Tech from the same branch is the best option because in that, you have a vast experience in that field that would be beneficial in every form.

Aeronautical engineering is niche branch of engineering with lesser scope in comparison to mechanical engineering. If you are interested to work in the field of aeronautics only and sure about your interest area then B.Tech and M.Tech in Aeronautical or Aerospace Engineering is a better option in co