Even though its 70 Attack versus Toxapex's 63 is not all that great, it has access to better attacking moves, like Lunge, Liquidation (late ingame though) and Poison Jab (TM) as well as Crunch and Bug Bite.
Toxapex is stellar defensively, deemed immortal by those who can play it well (and those frustrated by battling against it), with Regenarator and Recover. It is meant to tear away at the opponent, slowly, dealing passive damage. For it to deal decent active damage, it needs Merciless + Toxic + Venoshock, which which Toxic falls under passive damage as well.
So for the meta as well as in game, Toxapex is more defensive, whereas Araquanid is more active. In game, there's not much difference sometimes because opponent attacks are weaker and can be either Physical (mostly) or Special (slightly rarer), so Araquanid sometimes feels the heat defensively whereas Toxapex laughs it off.