I took both the JEE and CAT successfully (I am a parent). I understand the question as "What takes more effort, JEE or CAT?". The answer is clearly JEE, and by a wide margin.
JEE requires systematic and consistent study and practice for many months. So there is a lot of effort involved and a lot of preparation time needed. CAT asks questions on Verbal Ability, Logic and Reasoning and Quantitative Methods. The first two capabilities are built by a natural process over many years, depending on your reading habits, lifestyle, interests and intelligence. A few months of study does not alter those capabilities too much, except in terms of speed. Quantitative Methods are of Class X or XII standards. All you need to do is build up speed. CAT really is a test of your management capability - time management, prioritization, focus on strengths, concentration, breaking down complex problems into smaller manageable ones and so on. The CAT paper is such that almost everyone can solve all the questions correctly, but very very few can get them all correct in the given time.
I spent 4 months preparing for the JEE. And one week for the CAT!
Anup Varma
Bangalore