It all depends. There's such things as "Tankfenders", I believe which are Dark/Sonic or Sonic/dark, one of those two. So if you add that with solo damage bonus and assault numbs., it might be better overall.
http://www.virtueverse.net/wiki/Shadow_Mokadara
Depends on the defender primary. For mass x8 AOE a traps/ice is great. great buff/debuff numbers and also very very nice aoe dmg.
For ST then /sonic would be good because of the insane defender debuff values attached to that blast set.
Fire is getting ported but I heard ROF is getting towned down =(
Corruptor.
Corruptors are Defenders with Damage. They are much more soloable OVERALL.
Dark/ Defender is the most soloable so it is possible to tweak things and do various machinations to make a strong solo Defender - and that right there should be your answer. Corruptors kill things out of the box, Defenders don't.
Aside: I'm speaking generally. Generally, Corruptors do more damage and are more soloable. This is not an invitation to "Please come up with a Defender combo with IO sets that is better at solo." OP has been playing for less than a year, let's speak simple.
"Depends" does not answer the question.
Main reason that I'm asking is because I'm changing my lvl 20 AR/Traps into either a Beam/Traps or a Traps/Beam once Freedom goes live. I've heard that defender traps is superior due to the modifiers but I really don't have enough corrupter experience under my belt (mostly play blueside) to fairly judge the archetype.
Devs would post more if they could say "hi!" without people whining because they wanted them to say "hello".
-Nethergoat
The solo damage boost Defenders get will be superior to Corrupter scourge for minions and lieutenants - bosses and higher, it begins to get more interesting, as they will be alive for longer when scourge can kick in.
For elite bosses and AVs, scourge is clearly superior. There are threads in the forums analyzing the whole issue, but that's the back-of-the-envelope summary.
So to some extent it depends on if you intend to face a lot of bosses solo. If so, the Corrupter may have a real advantage.
I'd make the decision based on what you want the focus to be - especially at the early levels. Are you a beam rifleman with some deployable traps, or are you a traps-master with a beam rifle?
Corrupters will get more beam rifle attacks sooner, and Defenders will get the better traps sooner. Especially at the lower levels, the earlier access to more attacks will make more difference than the inherent powers will in terms of defeat-rate.
Another possible question is what you intend the character to be when "complete" - if you're going to try for things like soft-capping defenses, the higher Defender numbers for Maneuvers and the force field generator may influence your choice.
There's also something satisfying about seeing the "scourge" come up - even when it's overkill and didn't really do anything. 
But at the end of the day it's about making the character fun for you to play - so pick the AT that better matches your vision, and how soon you want access to the various powers. Either one will be perfectly capable of soloing CoH mission content, and will be capable of supporting a team when you're inclined to do so.
Synchrotron, level 50 Radiation/Radiation Defender
Fighting crime on Champion since 2004
I have two characters that are nearly identical, other than one being a Corruptor and one being a Defender. They both have Dark Blast, Dark Miasma and Power Mastery.
I can tell you that all the things that seem to make common sense from a basic comparison of the AT modifiers hold true for those combinations. The Corruptor is more damaging, the Defender is safer. At times, that can mean that the Defender actually solos faster, because she has to spend less time/effort trying to survive and/or manage enemies. In situations where the Corruptor's mitigation abilities can keep things under control, she solos faster, because she tends to do more damage, especially against LTs, bosses and EBs.
Blue
American Steele: 50 BS/Inv
Nightfall: 50 DDD
Sable Slayer: 50 DM/Rgn
Fortune's Shadow: 50 Dark/Psi
WinterStrike: 47 Ice/Dev
Quantum Well: 43 Inv/EM
Twilit Destiny: 43 MA/DA
Red
Shadowslip: 50 DDC
Final Rest: 50 MA/Rgn
Abyssal Frost: 50 Ice/Dark
Golden Ember: 50 SM/FA
Which do you consider better?
Since I normally solo or play with two other friend max, I'm trying to decide which one is actually easier. Namely, is the corrupter base damage and scourge worth more than the defender's solo damage bonus + higher assault numbers?
Devs would post more if they could say "hi!" without people whining because they wanted them to say "hello".
-Nethergoat