Neither of them are that great in-game, as playthroughs are more about which Pokemon outspeed and do the most damage. In that sense, there are offensive, speedy powerhouses that outclass these two. Some of the more prominent are:
Grass types
Steel types
Water types
Poison types
Neither Ferrothorn or Toxapex can do a whole lot of damage quickly, so they're inefficient in-game. If you're absolutely dead set on using one of the two, though, you should go for Ferrothorn. Steel types are defensively quite valuable, and there are a plethora of other Water types you could choose over Toxapex. Ferrothorn is also capable of dealing a bit more damage than Toxapex, with access to Iron Head and Power Whip. If you must, here's a moveset:
Ferrothorn @ Miracle Seed / Leftovers
Ability: Iron Barbs
- Iron Head
- Power Whip / Seed Bomb / Energy Ball
- Return
- Curse