Which is better for an MTech in electrical engineering, NIT Trichy or IIT Guwahati?

The choice of the institute for pursuing an MTech in Electrical Engineering depends on what you want from your Masters. 

• For those on the lookout for placements, NIT Trichy has a good placement scenario, especially for Electrical Engineering. They are even better than some well-known IITs in M.Tech, especially Power Systems. 
• However, if you are willing to pursue a career in research after MTech (Ph.D.) then you will have to give great importance to the choice of the institute. An IIT is best suitable for prospective Ph.D. holders as they have global recognition too.

Ultimately, the better institute is a matter of preferences and career prospects.