Which is better for mtech in mechanical engineering between nit trichy or iit guwahati?

The better option depends on what your purpose and life goals are and what you want from your Master. 

If you want to pursue a career in research (PhD) the best option for you is IIT Guwahati. Being one of the oldest and most highly reputed IITs, it has high credibility and will fetch you Ph.D. interviews much more easily. Additionally, IIT Guwahati also offers immense exposure through foreign exchange programs. IIT Guwahati has collaborations with various reputed universities and can be preferred over NITs. 

However, if it's the jobs you are after, then NIT is a better option as it’s more accessible to employers. Still, IIT Guwahati will always be my first preference. Be it a job or higher education, IITs always give an edge to other engineering institutes.