Neither is better!
Radation/Radation will be better at taking down AVs from my experience. And AVs are the name of the game level 40-50.
Dark/Dark, however, is very good accuracy debuffery, and will have the edge against mobs in terms of protection.
/Radation has the slight edge in AoE damage (particularly irradiate) and probably a slight edge in control and damage overall compared to /Dark.
Dark will have an easier time solo with the pet at level 32.
Overall, however, they are both fun.
As Cognito says, neither is better - but the sets have different ways of being effective.
Probably the main difference between Rad/Rad and Dark/Dark is that Dark/Dark concentrates on debuffing one particular thing - Accuracy - and Rad/Rad is perhaps a little more versatile.
Mind you once a well slotted Dark/Dark Defender - played correctly - gets going your team will really, really benefit from it.
The level of Accuracy debuffs from the set is simply awesome and if you add in a very strong Heal (second strongest in the game) and great control qualities you have a very effective powerset.
The only thing I would disagree with from Cognito's post is the statement about /Rad having the edge in control.
I would have to say that Dark/Dark has more control qualities.
The attack chain below will lock down and debuff a mob very nicely indeed
Fearsome Stare - Tar Patch - Tenebrous Tentacles - Darkest Night
However ... as Cognito says /Rad (Lingering Radiation) with it's -Regen is excellent for fighting AV's, which are littered right through level 40 to 50.
Dark Miasma also has -Regen, with Twilight Grasp, but it's not as strong as Lingering Radiation's.
What I would say is that whichever of the 2 sets you actually choose, you will be highly effective in a team situation.
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Sorry by saying /Rad has more control (very slightly) I was comparing to /Dark, not comparing Rad/Rad vs Dark/Dark.
BUt its really on comparing a superb single target stun (Cosmic Burst) vs the weak AoE disorientate and strong AoE immob that dark has. So not a whole hillabeans in it.
Comparing Rad primary vs Dark primary, id say Dark has the edge in AoE fear and single target hold. However, Choking cloud is a simply awesome AoE hold (actually better than dark) which is hampered by the risk in using it (i.e. in the middle of melee) and end cost.
But, differences are generally pretty small.
Howling Twilight is also an aoe disorient which can be stacked with dark pit, it's not just a rez 
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Its a good'n for sure. But rather long recharge. Fair call tho, forgot about it.
Choking cloud is still the king of defender holds if you ask me. And it needs to be, given end cost and danger it puts you in! 
Keep going with the 22 as i think you will level to 50 sooner with it.
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Its a good'n for sure. But rather long recharge. Fair call tho, forgot about it.
Choking cloud is still the king of defender holds if you ask me. And it needs to be, given end cost and danger it puts you in!
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Choking Cloud is an excellent power but I have to say that Fearsome stare just shades it overall.
Fearsome Stare is a ranged attack (less dangerous to use) and has considerably less Endurance issues but gives a very similar result when used.
It's only my opinion mind you but I'm basing it on experience from using my 34 Grav/Rad and 50 Dark/Dark.
I do agree with Shannon though. Stick with the Rad/Rad at 22.
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There's only two words needed to answer that question.
Fearsome. And Stare.
Seriously, I play a Dark/Rad def for a small themed team (two scrappers and a blasters, besides myself), and I often feel like there's nothing I can't accomplish. Things just don't hit us. They're too busy quaking in their boots most of the time, and the rest of it their accuracy is so low that they flail pathetically and miserably at us. Drop Tar patch and those left able to run suddenly can't, and everyone's resistance drops very noticeably. By the time that's done, Fearsome Stare is up again and the next mob awaits.
It's sillygood fun really, and I'm only level 22 so far.
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Fear is great, but isnt equal to a full blown hold. Hit em with an AoE and they all get a chance to pummel you back.
As with all things it depends on the team you are with. With plenty of AoE being flung about (especially things like AoE toggles) fear becomes a bit sub-par compared to a hold. IIRC it does give yet another -acc debuff which is darks strength.
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Fear is great, but isnt equal to a full blown hold. Hit em with an AoE and they all get a chance to pummel you back.
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Fearsome Stare does have a to hit debuff in it. Stacking to hit debuffs from the set means that it quite often doesn't matter if they try to pummel you back. Combining Fearsome Stare, Tar Patch, Tentacles & Nightfall allows my D3 to solo groups of 6-10 even level mobs with AOE attacks without fear of being hit.
As I thought.
End of the day both sets are pretty nifty in the right hands in the right circusmtances.
I wish to know which f these two combinations of primary /secondary is better / more effective por play.
I only PvE and mostly on teams.
I got a Dark /dark at level 7 and a Rad / Rad at level 22... as I have not unlimited time to play and can seriously level only one of them, I would like to read some opinions about which is better at high levels.
Also, some builds would be really much appreciated.
Thanks!!