Science

None of them answered your question correct. So, at first about the placement: No IIT's have got 100% placement but only those opt for campus placement which is around 60%-70% of the total eligible candidates. So, you can't be placed if you don't opt. So, I don't see a problem in placement in IIT here. So, don't believe those misinterpreted placement statistics in your newspaper. (Note : My words are coming from the official placement data published by IIT Bombay. )
Now, about IIT or IIST. You should go to IIT if you wish to continue your study and do M.Tech or Ph.D. as when you apply for higher studies the IIT name will give you an edge all over the world. I know that IIT's don't have good ranking on those lists but trust me people know and admire them (I have read about it on Quora that many of students say that many of their best batch mates were from IITs, Stanford, Harvard, MIT students. Sundar Pichai would be the most prominent example). Now, if you want to do job after B.Tech then IIST is better as you get a job in ISRO/DRDO directly but bear in mind that only those who score best in all 8 semesters will get a job with more that 7.5 gpa in each semester, which means that just getting more that 7.5 gpa won't ensure you that job and if you score less than 7.5 you are not eligible (you won't get kicked our something as someone answered. Nope, you have got plenty of other options. )
Hope it helps and that I cleared all the doubts and wrong answers which were given. If not then ask your question in comment section. And best of luck for your future.

You can join some research organisation, or companies into research, or clear GATE/NET and join Ph.D. with a good lab. Also you can join for as a JRF in a lab and enroll for Ph.D. Next best option is to take up a course in clinical research/ medical coding and getting yourself employed more rapidly. Both the industries pay you handsome salary. Actually the former is the equivalent of IT jobs in our industry. There is a lot of scope and if you dedicate yourself properly in a decade you will find yourself in a great position career wise and finance wise. The next best option is to identify a problem and solve it through a start up.
Also, if your college/ university as an incubator scheme apply for it. They will know you better and assist you accordingly.
All the best.

Physics is a very good subject to read, no doubt about that. However there are few things that one must keep in mind before pursuing a higher degree in Physics, such as Master's degree. Choose the university wisely, there are numerous universities in India providing M.Sc. Degrees in natural sciences subjects. But it is advisable to choose your university after evaluating factors like, alumni, faculty, laboratory, placements, research papers etc. For example, student who has a post-graduate degree from University of Delhi has many chances of getting into Engineering and medical coaching institutes. In fact institutes like Akash and FIIT-JEE go for campus placement in University of Delhi, IITs and other major ones. Decide what your plan is after M.Sc. Because most of the students have no idea what they are doing and they take admission anyway. You may be very good with physics but keep in mind that there are many like you in the university. Decide beforehand that if you want to go for Ph.D. or you want a job. I have seen many guys start to prepare for government graduate level exams and PSU banks after completing their post-graduation. If you really have the dream of joining government services start to prepare today than waiting for another two years. Read well during your course; once you have decided to take admission in M.Sc. Physics take the bait and give hundred percent focus to what's in hand. If you do well, there are good chances of clearing JRF, NET and other such exams. If you score well in these exams you may get into premier institutes like IISER for pursuing Ph.D. Over there stipend is decent, INR 42000 Per month. Once you have completed your Ph.D. from such a reputed institution there will be many opening for you as a scientist in government facilities like DRDO and all.