Is it wise to prepare for the ISI Kolkata B.Stat entrance exam instead of IIT?

Taking a course at IIT is just as difficult and demanding as pursuing a B.Stat course at ISI. They are whatever you choose to do with your life, whether you like mathematics/statistics enough to spend three years learning statistics (and a little math) and statistics solely at ISI, or you believe you are skilled in another subject that has a good course at one of the numerous IITs.

It depends totally on where you want to be. Furthermore, even if you prepare for college X, there is no guarantee that you will be accepted. As a result, keeping colleges Y and Z in mind as a backup is always a good idea.

Only you can decide which option is preferable for you and what you want.