Here I would go with it branch at nitk as it is one of the top colleges in india renowned for btech also it is amongst the top nits of india so you will have a great exposure as well on the other hand iiser kolkata is also a renowned institute ut in the field of science so ultimately it boils down to you that what you want to do whether you want to do research or do a btech and become a computer/it engineer hence choice is upto you.