Hello shubham

there is no any disadvanatge of doing a Mtech after Btech.you get a more knowledge about subject. if you score a good marks in gate  .there is probablity for selecting in govt sector.

mtech will help to  heigher promotion.if you working in industry

thanku

Hello

Do Mtech only and only if you are intrested in studying for whatever stream you are getting for Masters.
One of my friend at IITKgp once told by his prof in his first lecture, "stay here only if you want to study seriously, btech is enough to make money".

So only advantage is that you are now some kind of expert in your field and is more eligible to go into research.
Mostly all studious who went to IITs for Mtech have also opted for PhDs.

After Mtech you are eligible to be a faculty and spend your life at college again.

I hope it will help you
Good luck...