Our guild is prepping up to start SSC soon and I will be on add duty for hydross.
I've been reading around and have been getting mixed answers to how i should gear myself.

Some say go def and get whatever resist you can after crit immune (which would be a loss in resist)

Others say get max resist and not worry about the def.

Currently if i go 405def(for a lvl71 crit immune for a druid) i go down to about 150 NR/FrR(unbuffed)

Going with total resist i get 360 def(Earthwarden adding the 10) and ~200 NR/FrR(unbuffed)

Which has worked best for guilds that have downed hydross? and at what amount each?

As an add tank I would go for as much resist as possible, it's not important to be crit immune at the adds as you can stun them should you get a spike. You'll definatly mitigate more damage by having good resists than being crit immune, ovet the ten minute timeline.
Also note that you can pretty much OT without any resist at all, on our first kill we had a druid tanking without any resistance gear. I'd say it's mostly a healing issue if your tanks are dying.

I disagree. Reaching crit immunity is insanely easy for druids anyway and it's the spikes that will kill you in this encounter rather than lack of healer mana. An unresisted crit from an add at the 250% mark is going to seriously hurt (~17k), and unless you reach a certain amount of resistance (i forget how much exactly, around 400 at lvl 70?) you're always prone to completely unresisted hits. I'd sooner go with zero resistance gear than no crit immunity.

The itemisation is out there anyway. Personally I use resilience to make up most of the crit immunity (timelapse shard, glad maul, +15 enchant on chest) with the rest from defense (heavy clefthoof vest + legs, also good for stamina). Apart from the craftable neck/rings, the rest of my gear is lvl 68+ leather/cloth "of frost/nature resistance" greens with stamina enchants where possible.

Raid buffed I end up with over 20k hp, crit immunity and 200+ frost/nature resistance which is more than adequate. Actually considering dropping some res gear in favor of more +hit and AP to make holding aggro a bit easier and more predictable.

I must say, I've never heard or seen any strategy involving keeping Hydross adds alive to the 250% mark.

Regardless I must agree with Nietzsche, high resistance in both schools is way more important than being crit immune (under the assumption the goal is go kill the adds fast to get as much time on Hydross before the switch). A crit within the 2 first marks (30 sec aprox.) is nothing , damage wise, compared to the amount of mitigation.

One way to do it is to just focus on 1 resist type while beign crit immune and keep 100-130 unbuffed for the other. Tell warlocks to shape the adds and alternate with the other tank depending on the phases.
Lets say you focus on frost... On a frost phase you both tank the 2 initial adds regardless of resist. The other tank with heavy nature tanks the 3rd while you with heavy frost tank the 4th.
Roles swap for the next phase.

This works wonders for tank survivability and can give you extra dps since you can off-tank the last add and dps it while hydross is repositioned for the next phase.

We have enough Hydross threads.