Ya exactly why dual crypt gargs > hunts; because you can focus fire like 20 on the same unit, best you'll get with spiders is focusing 6 on the same unit and even then he just walks the unit away to his well and ****s up all your fiends!! No doing that with gargs. Also hunts can't even hit gargs, so they're obviously better counter than fiends. Also with gargs you can (and should) go to their base/xpo and harass their wisps. With ground units they can well block wisps so you can't hit even one, but with gargs you can harass all day especially if they went hunts. Also dryads cost lots of wood, so with successful harass you can easily get 5 wisps which is very successful harass and will crimp his dryad production. Also with gargs, by limiting his wood and therefore dryad production, you can keep your garg army larger than his dryad army easy because gargs > dryads (you should have gargs upgraded).
But in general, mass spider > mass hunts because of: A) DK Coil, B) Range, C) Damage...but if you open ghouls and he goes hunts and you cant stop him or at least slow him down (kill hunt hall as it goes up, or moonwells or something), then chances are you won't be able to get any kind of advantage by switching to spiders on your way to tier 2 (or whenever). In which case if NE mass hunts on you, make a tower and mass gargs.
You could of course open with spiders and creep...but again, like I said archers will wtfpwn your spiders...and if they are accompanied by DR...then GG...(unless they suck and let you make it to destros)