Mechanical Engineering

Hello,
Pursuing Aerospace Engineering as a specialization in Post Graduation (M.Tech) is indeed a better choice. Aerospace Engineering is a niche branch in India with limited job opportunities at present. However, since you already have completed Bachelors in Mechanical degree, you can apply for jobs in mechanical sector, whilst you can also keep looking for jobs in your desired role in Aerospace Engineering. This way, your career won't halt. While, your job in mechanical domain will keep earning you experience, you can choose your path in the Aerospace domain whenever you get a good opportunity.
I hope this answers your question to your satisfaction. If you have any further questions, kindly revert.