See both the institutes you have mentioned above are the elite institutes of India. So they both have some strategy to select students to keep up their elite status.

So both the exams have some level of difficulties.

In case of JAM to secure a rank under 50 , you have to study a lot and have to solve questions as fast as you can. Also you have to be intelligent enough in identifying the lengthy question given in the question paper to save time. So in JAM the problem solving skills as well as time management are checked in the examination.

Now in case of ISI exams problem solving skills along with theoretical knowledge are tested. In the question papers they include both subjective and objective question so you have not to worry about time management but about the accuracy. After their written examination they conduct interviews to recheck the explanatory ability in a student.

So both the exams have challenges in their own. But as ISI have interview then you will have to work hard regarding that too.

I hope my answer helps you. All the very best for your future endeavors!