The Spider Staff does more damage, but the minions are most ideal in closed spaces or large arenas with back walls. You can see their limitations quite clearly in fights involving anything that can be in the air. The Twins, Skeletron Prime, The Destroyer and its probes, The Flying Dutchman, Martian Saucer, Wyverns, etc. These enemies must be fought in an arena with walls so the spiders can actually reach them, but they are also pretty slow and are not always successful. Enchanted Daggers, on the other hand, are very fast and can fly towards their target and thus they don't need any terrain preparation to properly utilize.