If you're going to solo, Sonic/ doesn't bring a lot to the table as 5 of your powers require an ally (though Sonic Repulsion is usually skipped, so really 4).
Traps/ is a really solid soloer and can team well too, you just have to modify your playstyle depending on which you are doing. Another really strong choice is Time, it's a very up-in-your-face primary for defenders.
As for Secondary, might I recommend /Sonic? the -Resist brings a lot more to the table than Fire's straight damage for a defender.
I have a Time/Sonic that I usually solo but I know he teams really well as well.
If you want to go Poison (It's a solid set) I'd recommend going /Water Blast so that you have a self heal to help yourself solo. My main experience with poison is a Plant/Poison controller, but I'm currently working on a Poison/Water Defender now and he's solid.
Storm is a ton of fun, but most of my experience is on the Controller side again. I do have a Storm/Dark that's around level 30, stacking AoE Stuns (they're only Mag 2 each) is a great option for mitigation, and Tenebrous Tentacles works well to keep things in range of Freezing Rain.
Ok, I don't know if I've helped or given you more problems, but that's just my 2 cents.