The spider will win every time, think about it... Slug. Slow moving, slimy ugly, eaten by a variety of other animals and birds, Often accidentally squashed, no one wants to save it because it has very few redeeming features. It's a pest, eating veg that you plant in the garden and is easily defeated by pouring salt over it.
Spider. Fast, dangerous and comes in all shapes and sizes, it can be poisonous and can kill people... I mean, for a creature so small that's pretty fucking amazing. It spins webs, which are natural art, is actually a gardener's friend as it feasts on insects that might be considered pests. On top of all this it also wins in the leg contest, racking up 8 of the buggers compared to the slugs paltry none.