One simple yet concrete reason to choose IIIT Delhi over DTU or NSIT is the quality of education it offers. IIIT Delhi is known to have some of the most highly experienced faculty, top-notch infrastructure, and a well-updated curriculum. Many students from other colleges, even IITs, often come to IIIT Delhi for internships.
As far as placements are concerned, one must understand that getting a job after 4 years is important but not more than a successful long-term career. That being said, the quality of education affects your long-term career largely. You may or may not get placed in a big MNC if you join IIIT Delhi, but in the long run, you will certainly become successful. Also, the statistics show that NSIT has better placements than IIIT Delhi but in reality, these figures are inflated because of one or two international packages.
So, it’s best to focus only on the quality of education, and you will see how a long-term successful career unfolds itself before you.