The salary is entirely dependent on the subject and the proportion of knowledge you have.

Now PhD or Mtech is a very relative term to ones future prospect. If some one is really interested in research then he/she should go for PhD. But if some one is interested in higher studies but just one to do specialization in a field then he/she should apply for Mtech.

So for your career goals it may differ. So you have to decide wisely what to do and take your future path. PhD will offer you a monetary support during your phD period but MTech is a paid course or stipend is very less in comparison to the fees.

But the interest in the subject is the thing what is required.

Have a great eve!