Both are amazing waterfalls so you can’t really go wrong. One is much harder to access, though, so maybe that can help you decide. You can hike to the Nauyaca in a little over an hour and the trail is mostly flat with a few steeper spots, especially at the end. Getting to the Diamante is much harder and you would want to devote a full day. It takes a few hours to get up to the area where the cave/twin waterfalls are and the incline is much steeper. You also need a guide to access the property for the Diamante so it is more expensive but don’t for the Nauyaca. If you haven’t seen them, we have blog posts on both with more info. Here are the links:
Hope that helps!