The first difference is that the Polytechnic offers the course of Diploma in Engineering where as in IIT's you will get to have higher degree such as BTech, MTech etc. Apart from this IIT's are better institutions than Polytechnic. They are the top institute in India which offers great academic and placement to its students. So if you are interested in Engineering and want to do a BE/BTech program in Engineering and want to get placed to top notch companies, then IIT's is the place you should aspire for. The admission to the IIT's are provided on the basis of JEE MAINS & Advanced examination.

To know more about the Top IIT's in India, visit the link below:

https://engineering.careers360.com/colleges/list-of-top-iit-colleges-in-india

To know more about Polytechnic, visit the link below:

https://engineering.careers360.com/articles/polytechnic

I hope this answer helps. All the very best for your future endeavors!