The great thing about Storm Defenders is you can do a lot of crowd control. Storm is a very difficult powerset to play, as you have to be constantly looking left right and centre, repositioning and debuffing mobs.
Great powers I definitly recommend you take: Snow Storm, Freezing Rain and Hurricane.
There are a many great combos to use... eg: Freezing Rain them so they are debuffed and being knocked-down, then snow storm the lieu or boss, then just before the rain's about to run out, run in and thunderclap to disorientate the minions, then.... you get the idea. 
Sonics is a great power set to debuff the damage resistance of enemies, how many powers you take from it is up to you, it all depends on if you solo a lot or want to contribute to a group. My character is team-orientated and I've taken only a few attack (to stack the resist debuffs), plus the screech (single target disorientate) to stack with Thunderclap.
I've Storm as my Alts secondary so I can give a little advice.
Gale is mostly worthless. You get the choice so go with O2 boost. It's not great but better than Gale. You'll get Hurricane for knocking mobs away.
Snow Storm reduces a targets movement speed AND attack rate. The Attack Rate bit is the useful bit. Don't bother slotting with extra Slow enhances as they only affect the movement speed. Slot with Endurance Reduction instead. And watch it if the target runs. It can give extra aggro.
Freezing Rain is your Big Hitter. I'd always let the Tank get aggro, Snow Storm a minion at the back and then Freezing Rain the group. The reason I do it this way is because Freezing Rain will tend to scatter the mob. Anything you can do to prevent or slow this is useful. Put -Damage Resist enhances in it mostly.
Steamy Mist is a useful power that gives 20% damage resist to Fire, Cold and energy (I think). I'd always have this on. So slot with end reduction and +dmg resist enhances
Hurricane is great but you'd generally not have it on all the time. If a group is in a dead end you can use it to keep them in there, in the freezing rain, and on their backs. You can also "brush" mobs with the edges of it to apply the -accuracy debuff to them. You can see when a mob is affected.
For a Storm Defender I'd get Thunderclap as a control power. But I never did with my main.
Thunderstorm I find useful. I'd slot with 3 dmg, 1 accuracy, 1 end reduction and 1 range, as its static. Its Knockback means you could set it up in front of the other ranged squishies to keep enemies away from them.
thx for the help u 2 
Freezing rain and huricane is a top combo. esp in indoor missions, bung FR in a dead end or corner, then heard all the mobs into it with huricane.
And just for fun you can wander round outside, knocking mobs off of rooftops in most maps.
Yep. Hurricane is so much fun with the Rag Doll graphics now.
Oh and Tornado I find awful. Some people like it, espcially solo, but I'd prefer to be ground up and fed to pigs rather than pick it. At the very least go to the Test Server and try it before buying it.
If you use it in a group you'll probably be shouted at. Unless its a really panic situation. Even then I'd take the fed to the piggies option.
iv started a storm defender with sonic secondary i quite like it BUT dont no how a storm defender should work any advice ild be gratefull