Hello Sankalan,
You should go for mechanical engineering branch among these two options.

Mechanical engineering is always the evergreen branch of Engineering. Electronics and Instrumentation is also good.
Now you have to think, what is your goal? What do you want to study actually.
Both are god branches. And placements are definitely better in Jadavpur university for both the brnaches than other government engineering colleges.
You have to decide what you want to study.
Good luck...