Dark/Rad imo. The one thing that dark doesn't give you is -Def and you can cover that with Rad blasts. It has better AoEs and burst damage as well.
/Rad, Higher AoE damage, and one of the few debuff types you lack otherwise.
Found Rad useful early on but not so much later and dropped half.
Dark is more controllery. You'll really mostly on your cones for AoE damage.
Rad will give you a better single target attack chain, some okish AoEs, and def debuff which you otherwise won't have.
So essentially, Dark is a bit safer, Rad is a bit more offensive.
Dead Calm's Defender Manifesto
Ty for feedback. Howdy Faulty, hope to see you again soon mate
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]
Why not roll it /elec?
Sure, it's not debuffing, but Short Circuit's pretty dang handy...
Not about too much at the moment False but you may catch me. 
Once I got Tactics I found that the -def wasn't that important anymore. Plus two of the AoE are short ranged and Neutron Bomb takes forever to animate (although I still took it cos it looked good). Sonic looks good for the extra offense and -res debuffs. Can't say I've ever seen one about tho....
Tar Patch is -res
Try Rad/Dark - awesome debuffs / buffs and dark's teneborous tentacles make taking down AV's easy (it stops them running away but has more chance of getting them than a true hold), damage output is pretty low until you get to the epic group so it's a team build.
Of course I'm not biased *hides signature* =)
I have a level 9 Dark/Rad Defender as my main Defender to complete my set.. finally decided that THAT was the one to work on, and in my opinion, it's very good for what it does -- and I'm enjoying it so far.
That and I've never seen a 50 or any of that combi on Union yet, since I've been here, so I thought I'd be different and make one.
I'm thinking of rolling a debuffer. Can't decide between dark/dark (extra to-hit debuff) or dark/rad (extra defence debuff). And no, I don't want to go do radiation(primary) instead. DM is uber
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]