I assume you mean EV3 robots - the NXT is the previous-generation Mindstorms brain, and hasn’t been sold new since ~2014. The motors and sensors in the NXT kit are different from those in the EV3 kit, but are compatible.
In the camps I’ve been running recently, the kids build and program both an EV3 robot and an IQ robot in the same week. We start them on the EV3 bots and then move them on to IQ after a couple of days.
IMO, IQ is the clearly superior platform on paper, but in practice there are various minor issues that make it a bit more even between the two. Some random thoughts:
- Kids LOVE teleop, and Mindstorms can’t do that very easily.
- IQ parts are physically much harder to put together and take apart, even for adults. On the other hand, we have some 3D-printed pin pullers which help a lot, but not in every scenario. We have not needed any tools to take apart Mindstorms parts.
- If you’re programming on iPads via Bluetooth, EV3-G for iOS offers only a small subset of the features of the desktop version, whereas Modkit on iOS has near feature-parity with the desktop version. However, the Bluetooth connection to the EV3 seems to be a bit more reliable than that to the IQ brain.
- Modkit will do a bunch of drivetrain-related math for you, providing blocks like “Drive forward X mm” and “Turn Y degrees”, whereas EV3-G makes you do all that math yourself (which can be a bit much for younger kids, though middle schoolers can probably handle it).
- The IQ super kit is about $90 cheaper than the EV3 core set, a difference which will really add up if you buy enough kits for ~150 kids.
Overall, the EV3 platform is somewhat less capable but quite a bit easier to use and more polished. If it’s feasible, I would recommend that you play around with each kit a bit before making a final decision.