Hwange vs Zambezi National Park

Hwange is Zimbabwe's largest national park, and it's known specifically for elephants — herds numbering in the hundreds aren't unusual, especially near the park's waterholes in the dry season. It's a proper multi-day safari destination, not a quick add-on.
Zambezi National Park is smaller and river-focused, running along the Zambezi itself upstream of the Falls — hippo and crocodile are a given, and game drives here tend to be shorter, easier to combine with a day trip than a dedicated multi-night stay.
If you only have time for one, the deciding question is usually simple: do you want a dedicated safari built around elephants, or a shorter, river-focused add-on to a Victoria Falls trip? Hwange answers the first, Zambezi National Park the second.
