I'm not sure I'd agree; I have been to both. I personally find them both very interesting, but for different reasons. I'd probably argue that Pompeii was better, just because there's more to see and more variety.
If a person could only pick one, they would have to decide whether they wanted to see how the wealthy lived (Herculano) or how regular folks lived (Pompeii). Additionally, Pompeii is a MUCH larger site, so if you're pressed for time or have mobility difficulties, that could help make the decision. Also, Herculano is a bit cooler (temperature-wise), so if it's hot out, maybe that's a deciding factor