Which tour should I take for the Natural Caverns tour? The discovery one or the hidden passage one? Which one is more interesting ?

I'd recommend both, just did them this last weekend. But if you must pick one, Discovery had more of a "Woah" effect for me. Although, it felt shorter. Hidden passage felt longer and deeper because of all the stairs.

I would do both

We did discovery and enjoyed it but plan to go back for the more involved tour

I loved both. The hidden passage one has a lot of stairs involved.

Both are very interested.

Discovery if you have not been before.