No, the IISER aptitude test isn’t as difficult as JEE Advanced. It has a similar difficulty level as JEE Main. The one thing that differentiates IAT from other entrance examinations at this level is that it imposes equal weightage for all four sections, i.e. Physics, Mathematics, Chemistry, and Biology. 

Questions asked in the Mathematics sections are conceptual but doable. The difficulty level is less than JEE Advanced but slightly above JEE Main. Physics questions are easier than JEE Main. You just need basic problem-solving skills, to score well. Chemistry questions are comparable to JEE Main level. Biology section is very easy to score. 

If you are truly passionate about Physics, be it theoretical or practical, you should choose either IISER Pune or IISER Kolkata. IISER Kolkata is the best option for Physics, while IISER Pune is well-known for its overall academics and infrastructure. Here are the reasons why you should prefer IISER over IIT.

• Most students joining IIT, aim to get corporate jobs. If you are interested in the subject and want to do actual research on it, IISER is the best place. You will get to meet like-minded people too. 
• You can check the faculty and the curriculum for Physics at IIT. Even IIT Kanpur which is known for its Physics department puts more emphasis on applied Physics. Most IITs leave out theory-oriented courses like ‘Cosmology’. Also, you won’t get the lab infrastructure similar to IISER at IITs. The labs at IISERs are specifically designed for research works. From the 5th semester onwards, the course curriculum at IITs become quite different. It becomes more technology-oriented. While students at IISERs are required to do a one-year-long research project.  

Based on the performance over the last few years, IISER Pune will be better than IISER Kolkata. 

Both IISER Pune and IISER Mohali offer good academics and placement opportunities to students. In terms of facilities, both institutes are on an equal footing now. The research output of IISER Mohali is catching up. In terms of coursework, electives at both institutes are slightly different. You can check out the course structure and faculty profile from the official websites. Before joining, you should also get to know about the culture of the places. You can talk to the current students from both institutes. Do your research about both institutes and choose according to your convenience.

Both IIT Kanpur and IISER provide excellent quality education but IISER has a slight advantage when it comes to the quality of research conducted. 

IIT Kanpur 

• IIT Kanpur provides you with the necessary exposure and brand name.
• The students can be seen as more interested in jobs than in research work. The environment at IIT Kanpur is not conducive to cutting-edge research work.
• The professors limit their teaching to the extent that students become excellent employees for outstanding companies.

IISER 

• No compulsory attendance rules yet the classes are packed with students.
• The professors are very learned and approachable. The quality of education they deliver is top notch and students are always approaching professors to get their doubts cleared
• There is great harmony between the juniors and seniors and ragging is forbidden. There are heterogeneous groups of students every year

IISER provides an excellent environment for students who want to become future scientists and contribute to society. It will also open doors for Ph.D.

There is no clear winner between IISER Pune and IISER Kolkata. IISER Pune and Kolkata are quite old and equally reputed. IISER Kolkata is more known for its Physics degree. While IISER Pune is reputed for Biology. 

Comparably IISER Pune has a better location. So, students receive better employment opportunities.