I would prefer Amoonguss. It resists Electric and is neutral to Ground, which are both good qualities in a format with Zeraora and Excadrill. Amoonguss can also reliably put opponents to sleep, paralyze them, or badly poison them, while Toxapex can badly poison or burn opponents. Toxapex has an arguably better form of recovery in Recover, but Giga Drain works quite nicely nonetheless.
Toxapex is better in some scenarios, like dealing with Volcarona, but I believe Amoonguss is a bit better in general.
Hope I helped!