Colleges with lots of graduates will always have a commensurate bump in rankings - do not put too much weight on those numbers. Both have good students and professors. Both would give you the chance for an excellent education.
Here are some other things to consider when looking at engineering:
- Four year graduation rates. The curriculum is rigorous and often takes longer than four years, especially at public universities where classes are impacted. You end up paying one more year of tuition and your career is delayed by a year. Big edge for ND in this case, 90% rate vs. mid-40s for Purdue.
- Job placement in your desired major. I couldn't find the statistics but both have excellent reputations.
- Other majors. The reality is that somewhere around 40% of engineering majors drop out or switch majors. What is available if you find it is not for you?
- Peer group. How would you fit with the other students? How happy are they, and how happy would you be around them? This is a lot bigger than people think. Happy people learn faster and do better. A four year struggle-fest is not recommended. Visit each, talk to students, and decide.
Are you a senior? Already applied?