The answer... Spider Riders (unless you're gacing Bretonnians, Dwarfs or anyone else with a high armour save)
Always take Spider Riders if facing high toughness enemies - it's the poison, it ignores toughness if you roll a 6 to hit (the enemy still gets an armour and ward save if applicable).
You spiders are not battle winners, they need to flank enemies and be used as support troops or artillery assassins.
Always take spiders (in numbers, ie a lot) if facing Wood Elves, they can quickly chase down Waywatchers and Glade Guard through the woods without movement penalty.
Note: they have an above average initiative of 4 aswell giving you a better chance of getting your poisoned attack in during round 2 of combat.
