In this post I will discuss each of the Druid builds, specifically in terms of how well they fulfill their secondary role.
Predators are secondary strikers, and with the right feats and items they can become quite good at it. Some good places to start focusing your character are either picking up Enraged Boar Form and a Horned Helm to charge as much as possible (like a Barbarian), or Ferocious Tiger Form and Claw Gloves to become a Rogue-like striker. Going the "rogue" route will give you a higher payoff early on since Claw Gloves gives you an extra 1D10 and the Horned Helm is only 1D6. Plus, between your controller effects and flanking you should have CA almost every round (whereas you won't charge every round, since encounter and daily powers aren't usable on a charge). As you get higher in level (by late Heroic or early Paragon), you have enough resources to do both. Charging an enemy that you have CA against will get you DPR (damage per round) that's well into striker territory, and you can set that up at least a couple of times per encounter most of the time. Predators also have better mobility than some strikers since they can shift as a minor (or free) action whenever they wild shape (in Paragon they can even shift # of squares = to their Dex mod when changing from humanoid to beast form), and many of their powers incorporate mobility as well.
Because a Predator's damage comes largely from items and feats, they don't need to sacrifice much control (as control, in all controller classes, largely comes from power selection). All of your Dailies should be control oriented (unless you opt for Summoning, which is still control but will also add appreciably to your DPR), and I would argue that all of your encounter powers should be as well (though 1 pure damage encounter power isn't going to sacrifice too much control either). Also, remember to look for synergies; for example 1) Savage Rend + Primal Wolf for good damage and the ability to eat enemy actions potentially every round, 2) Entangle for a good beginning of encounter setup then a DPR boost throughout the encounter from the expanded crit range.
Swarm Druids make excellent secondary defenders. They may not be as sticky as Fighters, but they're nearly as sticky as Wardens (both have at-wills that slow, and both have low level Dailies that can prevent shifting). With Hide Armor Expertise (it's inevitable that they'll take it), they get good AC in addition to having ranged and melee damage against them reduced. Add to that a steady source of THP in Paragon (through Bolstered Swarm) and various powers that make them even tougher to take down (though this will come at the expense of control), and they can be harder to take down than many defenders. I don't have experience with Swarm Druids aside from playing around with builds, but it seems like they're the best defenders of any class that shares that secondary role, and if built right I'll bet they can even serve as primary defenders if there aren't too many squishies in the party.
If you're looking to optimize survivability, you might want to consider making a Longtooth Shifter Druid. The racial power makes you even tougher, and it's augmentable with the feats Longtooth Spirit Shifting (Primal Power) and Beasthide Shifting (PHB2). Note that the damage reduction granted by Beasthide Shifting stacks with your Primal Swarm ability that reduces damage (since it's technically not "damage reduction"). The racial bonus to your Wisdom is perfect, and the Strength bonus might not go to waste either since you may want to pick up Powerful Charge (PHB).
I'd like to start out by saying that I think Guardians are sub-par. The description of the Guardian Druid makes it sound like it's going to be a good build; after all, with a secondary role of leader it's the most logical translation from the average 3.x Druid. Unfortunately, the Druid doesn't have a ton of good leader powers, and most of the ones that it does have are not Guardian specific (some actually have Predator riders). As sub-par secondary leaders (see above on the importance of a Druid's secondary role), Guardians become sub-par overall given the heavy emphasis that the Druid class places on secondary roles. Guardians aren't much better controllers than the other builds. To make matters worse, Guardians have the worst build-specific riders, often simply adding your Con mod to damage. And here's the icing on the very sad, stale cake: the Guardian's class feature simply lets you use your Con modifier to AC, which is exactly the same as Hide Expertise (feat from Primal Power). Thus, Swarm Druids can match your AC at the cost of a feat, and they still get their pseudo damage reduction class feature (likewise, Predator's get their +1 to speed and maintain AC through Dex as a secondary stat).
In my opinion, the best way to salvage the Guardian's utility is to multiclass Shaman and take the Mending Spirit MC feat from Primal Power. This allows you to use Healing Spirit 1/enc., and the Spirit Companion is in general a decent way to block a square (control). Also, there are some decent Paragon Paths that tend toward the leader role (Keeper of the Hidden Flame, Guardian of the Living Gate, and, especially, Spiral Wind's Ally).
Note: I may have been a little harsh on the Guardian Druid initially. Though disadvantaged with a terrible class feature (Con to AC) and some sub-par riders (which merely grant Con mod bonus to damage), they do have some decent riders that expand the forced movement of some of their powers. If you want to play a controller that focuses on forced movement, the Guardian's probably your best bet. I suspect that once PHB3 is released, the Telekineticist Psion build will probably fill the same niche, though I suspect the Druid will still have more/better zones, which complement forced movement nicely.
Summons intrinsically provide an interesting control mechanism in that they take up a square and offer HP resource management. Essentially, if the enemy forces attack and kill your summon then some (or all) of the damage dealt is net HP wasted (equivalent to wasted turns). This is because summons have HP equal to your bloodied value, but when they're killed you lose a healing surge (which is equal to half of your bloodied value). You can get even more devious by dismissing yoru summoned creature as a minor action when it's very low on HP, which effectively negates all damage done to it (since you don't lose a surge for dismissing). Thus, summons provide an interesting twist in contributing to your primary role, but this post is mostly focused on the Druid's secondary roles.
Druid summons most ostensibly contribute to the striker secondary role. This is because their instinctive actions (actions they'll use automatically if you haven't given them a command by the end of your turn) add a great deal to your DPR while the summoned creature is present. Simple, low maintanence, and a good choice for Predators (though any build will benefit from an increase in DPR). They can also be used as a flanking buddy, which is advantageous for the Predator that has damage bonuses when they have CA. Summons may also have some limited utility for the secondary defender (Swarm Druids), namely because they take up space and can therefore be positioned adjacent to an ally to prevent them from being flanked. Also, the Guard Drake is a mini defender in its own right and is an excellent choice for a Swarm Druid. Summons don't greatly contribute to the secondary leader role of the Guardian Druid, but the Guardian Briar does give allies a defense boost so at least that's something. The Guardian's focus on forced movement may also allow them to easily position enemies adjacent to summons, increasing the efficiency of instinctive actions. Note that Guardians will have slightly more durable summons due to their focus on Con (as opposed to Predators), and more importantly they'll have more surges available for when summons are killed. To summarize, while each build is capable of using summons effectively, they'll contribute most to the secondar role of Predators. This is balanced by the fact that Predators will have fewer surges to spend on summons than Con based Druids, though they can get around this by dismissing their summons when their HP is low.