After 12th both are good option in front of you when you are looking for the good course.

NDA is a civil service provider and IIT is one of the best institutes for engineering.

By joining NDA you can do more to serve your country defences and protect it from harmful forces.

And by joining an IIT and working in a job/entrepreneurship, you can serve your country in building its structure and economy.

IIT failed to provide a strong personality and character as an institute where as NDA came out strong on both

Eligibility for NDA:

Candidates pass in 10+2 with any stream for army.
10+2 passed with physics and maths in air force and navy

Age:
Age should be 16.5 to 19 years at the time of commencement of the course.

For NDA only boys are eligible.

Eligibility for IIT:

Candidates have done their 10+2 and also studying engineering.

Age:
Age should be above 18

But choice is you’re which one you select.