I use StoneSkin on must pulls for groups of mobs, and Strength of Earth on bosses. This unless our group is easily handling the pulls (and the tank is doing a good job holding aggro), then I'll use Strength.
I don't usually have much trouble choosing which Strength totem to use; usually it's Air that gives me trouble. With Wrath of Air, Grace of Air, and Windfury, usually as soon as I start putting one of them down, somebody in the group is asking me to put the other down. Recently I've been just dropping Grace and doing my best to appease the Windfury crowd with Flametongue.
Since I get conflicting requests sometime, I've recently made it pretty clear to the people I play with that I'll try to Totem Twist to their advantage (cycling totems to grant temporary buffs like Windfury and switching to another totem for stats temporarily). Totem Twisting used to be more of a pain because of mana issues, but with the buffed Mana Spring and Water Shield, it's become a lot easier, particularly now that I have a few points in Totemic Focus.
With good situational awareness, Totem Twisting can really do a lot for the group. I love quickly slamming down a Grounding right as an NPC caster throws off a spell, or a Wrath of Air and Mana Spring when I see that our healer is struggling, or a Tranquil Air totem when I see him pulling too much threat. As soon as the crisis is averted, cycle back the totems for DPS or defense. I wouldn't say StoneSkin or Strength of Earth are any better than the other, because it all depends on the situation... sometimes mid-fight I'll find myself changing to one or the other depending on how well the fight is progressing.
But, without too much more jabbering on my part, a good thing to keep in mind is that StoneSkin is of the most benefit when multiple people in the group are taking damage (or if your tank is having serious trouble surviving). Otherwise, Strength of Earth will benefit more people by upping all the melee users DPS, allowing the mobs to be taken down faster and subsequently reducing the total damage they end up dealing.