If you prefer more isolated areas, Pololu Valley may be a better choice for you.

Waipi'o Valley is an incredible place of arresting natural beauty and huge cultural and historical significance, and it's certainly worth seeing in its own right. However, it is a major tourist destination with lots of touring companies bringing visitors here from Kona and Hilo daily, and plenty of guides and guide companies there to help people make the trek down to the black-sand beach.

Pololu Valley, in contrast, is visited less frequently, and most visitors don't even know that you can hike down to the beach from the lookout point. There's a black sand beach there, as well, and there are steep sea cliffs just like Waipi'o.

Of course, Waipi'o is the better hike. It's about 6 miles RT with a 975' elevation gain, while Pololu is only about 3 miles RT and a 300' elevation gain. Waipi'o is also home to the Big Island's tallest waterfall, Hiilawe Falls, at 1,400 ft, accessible via the Muliwai Trail.

It's a shame you can't see both! Waipi'o is worth the visit even if you have to ignore all the other visitors, kind of like seeing the Taj Mahal in India: you know it's going to be a zoo, but you just have to see it. That said, if you only have time to see one and you're really not into crowds, you may like Pololu better. I hope this helps!