B.S in IISc or B.Tech in top IITs?

B.S in IISc or B.Tech in any one of the top 3 IITs.
You want to go for astronomy in PG after completing an undergraduate course.
IISc is the best institute to study Sciences In India. Many people prefer a seat there and after that only they consider other institutes. But IISc offers courses in Science and more oriented toward research. Whereas IITs are typical colleges where they will get you typical engineering degrees which will be useful to get jobs.
To get a seat in any IIT you have to get qualified in JEE examination and get a good rank. IISc admission will be on the basis of an examination called KVPY. You have to go through this examination and based on the result you will be allotted a seat.
The world rankings of the educational institutes say that IISc is far above than all other IITs of India. As such getting a seat in IISc and studying there itself will give you a chance to do PG in astronomy ina reputed institute and may fetch a good job.
Astronomy is a subject which is in very much demand and research-oriented people will have more chances to excel in this field. So people who did their BS in this institute will have a better chance to have a better career in the research area.
So I suggest you give first preference to IISc Bangalore. Second priority only any IIT.

You should not be confused with the twin different routs adopted to secure admission in the astronomical field. Here are two approaches-
1) First of all, secure admission in B.Sc with the combination of Physics and Mathematics consisting of three year course and later go in for admission for M.Sc in Astronomy from IIT Indore or any preferred location within the IIT circle offering you a two years post graduate course.
You need to secure at least 60 percent marks in your graduation level apart from clearing the written and oral interview fixed by the admission committee.
You may approach even other universities as indicated below for your persuasion for M.Sc in Astronomy-
1) Osmania University
2) Punjab University
3) Birla Institute of Technology, Pilani.
4) Tata Institutions of Fundamental Research Pune.
5) NIT Trichy etc
During the selection process, the Interview Board would examine the following potentials of the aspirants-
1) Solid knowledge of fundamentals of Physics and Mathematics
2) Must have the knowledge of the computer programming
3) Ability to work within the team
4) Good observation skills
5) One should have the ability to solve complex problems and conducting Research.
The other option would be to acquire a degree in Engineering from one of the best institutions including IIT or NIT with the following branches -
1) Electronics and Telecommunications Engineering
2) Computer Science
3) Instrumentation Engineering etc.
Now to be inducted in the M.Tech program in Astronomy, you need to secure an excellent GATE score not less than 97 percentile so that you may be admitted in the Indian Institute of Science, Bengaluru or in the Indian Institutute of Technology of your preferred location.
So in your case persuasion of M.Sc in Astronomy from the reputed institutions or taking up M.Tech from IISC Bengaluru after securing an impressive GATE rank after your engineering would offer you same value.

Astronomy is a fascinating area and there are not only good career options in it but there is ample scope for research as there are so many mysteries about the universe and heavenly bodies yet to be unravelled by the science of Astronomy. Before going into the query further, I would like to suggest here that if you have interest in Astronomy then inculcate it as a hobby also as star gazing and amateur astronomy is something which is very interesting and can be carried out with an ordinary telescope or even with naked eye in a clean environment outside the city limits avoiding the dazzling street lights. Making a career in Astronomy against a backdrop of amateur astronomer is an interesting proposition.

You have asked comparison between doing B.Sc. from IISc and that of B.Tech. from IITs. I suggest that if you have a desire to excel in the field of Astronomy then IISc would be a better option. Though there are many other options also and many colleges and universities are there from where also one can go up in this career line. Only thing is one has to do the commensurate hard work in this direction. One should not depend only on the top colleges as sometimes if admission is not got in them then one should not feel discouraged and can pursue one's dream from other colleges also. I strongly feel that top institution is not the only factors in one's growth and progress though it helps to some extent like campus placement, networking with foreign institutions for research etc.

To make a career in Astronomy the first thing would be to do B.Sc. with Physics and Mathematics as two essential subjects. These two subjects are the main knowledge pillars for learnings in Astronomy. After completing B.Sc. one has to apply for admission in some good college or university or scientific institute where there is a course available in M.Sc. (Astronomy). Some colleges are taking the students for admission to M.Sc. Astronomy course based on the marks obtained in B.Sc. and one should have at least 50% marks in that. Due to the tough competition one has to have very good marks in B.Sc. for this type of direct admission. Many universities/ institutions would have their own entrance test for these admission and one has to prepare for it thoroughly and if you want to see some model questions related to Astronomy then you can visit this page in ISC by clicking here and can navigate to other similar pages from there to get an idea of questions which can be asked in entrance examination. These questions are only indicative and you have to go exactly by the syllabus of the entrance exam of the institute where you would be finally applying.

Some of the institutions where one can try to get admission for pursuing this course are as follows -

1. Indian Institute of Astrophysics, Bangalore.
2. Indian Space Research Organization, Bangalore.
3. Tata Institute Of Fundamental Research, Mumbai.
4. Raman Research Institute, Bangalore.
5. National Centre For Radio Astrophysics, Pune.
6. Physical Research Laboratory, Ahmedabad.
7. Indian Institute of Science, Bangalore.
8. Inter University Centre For AstronomyAnd Astrophysics
9. Madurai Kamaraj University, Madurai.
10. Andhra University, Visakhapatnam.
11. Jawahar lal Nehru Technological University, Hyderabad.
12. Mahatma Gandhi University, Kerala.
13. Osmania University, Hyderabad.

B. Tech from the top Indian Institute of Technology (IITs) is the best option if you got admission in IIT or NITs.

IITs
IIT Bombay (Department of Physics) is one of the best.
IIT Madras
IIT Delhi
IIT Kanpur
IIT Kharagpur

Others
Tata Institute of Fundamental Research Mumbai
Delhi University

You have interest in Astronomy and for that you have to pursue B.Sc. in related subjects like Physics and Maths and after that try to get admission in some reputed institute like IISc or IIT. You have asked as to which one is better. I think that both are well comparable and It does not matter much as which one you are going to choose if you get admission in both of them. In my view it does not matter much whether you have your degree from IISc or IIT but what matters is your ranking there. If you get good score and good rank you stand a bright chance for research in a reputed institute and as well as there are opportunities of work association in foreign countries in the field of Astronomy. Many international collaborative projects are running in the world at different places and some scientists and research scholars from India are also actively engaged in them. Learning advanced Astronomy requires a good knowledge of Physics and Mathematics and you have to concentrate and focus in these two subjects in your graduation program.