Ondansetron is a strong antiemetic, and if you want to prevent vomiting in other contexts, then it's better than meto.
However, meto (and other dopamine-antagonist antiemetics like domperidone, or even some antipsychotics) also serves to increase the rate of gastric emptying, meaning the SN spends less time in your stomach and gets into your bloodstream quicker. This means that vomiting is both less likely, and less of an issue when it does happen.
Ondansetron isn't currently recommended by PPH, as far as I understand it.