So, why cunning over ferocity? Well, as a support you're not really focused on doing damage. Again, if you're gonna need the damage, because the enemy support is a high damage champion like Zyra, then go for it. But, if not, I always go for cunning and play the smart game. Of course, the resolve tree is a must.

CUNNING

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Wanderer : Helps you out. Also, Savagery is pretty much useless since you will not be farming.

Secret Stash : I think it's pretty self-explained. Assassin and Runic Affinity are pretty much useless.

Meditation : Your main poking ability Roar of the Slayer has a low mana cost (45). Still, you'll be using it a lot, and also your Soul Furnace is a bit expensive, and you also want it for poking and absorbing damage and aggro, so of course we'll take it before Merciless . That one, again, is not very useful.

Bandit : You're playing support, so this is a must. Once, long ago, getting money as a support was terribly complicated, but now, if you add this to your gold generating support item, you'll be getting your items in just the right time.

RESOLVE

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Unyielding : I recommend this one over Recovery because of the trades. Sure, health regeneration is important, but since your kit has a shield, and a bonus health infinite stack, I prefer this one. You will be able tu sustain more in trades and fights.

Tough Skin : Let's say you're going to all in your enemies at early levels (2 - 5). In this stage of the game, minion aggro can deal massive damage. This mastery allows you to tank it up, so you carry can get a kill. That, against the MS in brush and river from Explorer , is a better choice.

Runic Armor : Well, this is an easy one. Again, you have a passive that stacks you health infinitely, so you don't really need extra health.

Perseverance : You might be asking yourself, 'This guy just took armor and MR over health regen, why take it now?', well, it's simple. A 15% reduced cooldown for your summoner spells from Insight is nice, but your go-to summoner, Exhaust, is not like Flash, that has a very long cooldown. Of course it would be nice to get your flash a bit sooner, but you won't be using it too much, since your job is to take damage, and flashing in and out of fights or complicated situations won't be something that's gonna happen a lot. I mean, if it's happening a lot, you need to chill and play a lot more defensive. Flash if for emergencies, shut downs, very important objectives, first blood, aces.

Swiftness : I take this over Legendary Guardian simply because, even if both are useful, it's in my playstyle to protect my carry or other allies by acting as a meat shield, so I get CC'ed or focused instead of them. That way, I'll be going for more tenacity. But this is a playstyle decision: if you're going for early-mid game team fights, Legendary Guardian will help a lot.

Keystone Mastery: Bond of Stone : As a support, this is a must. This can mean the difference between loosing or winning your lane, and a fed ADC is every team's dream. Get in front, absorb damage, be close to your carry and focus your CC so he can just hit them like a machine gun. An ADC's job is not to stop doing damage. Make shure he can do his job. Grasp of the Undying can be useful for peel, whatsoever. But be careful when going for it. Also, strenght of the ages is even more health. I think it's not necessary, but hey - depending on the playstyle, all the three of them are useful in some way when playing Sion. You just have to figure it out by yourself.