You seemed to answer your own question. But I would add that Rad/Sonics have been around alot longer which means players have more experience with the combo. That's about it.
I have a Cold/Ice Defender and it always saddens me when I'm on a TF and someone says "We are going to need a Rad for the AVs."
I use to get fired up and give a few reasons why Cold = Rad and was great for AVs, but nowadays I just ignore them and move on.
I guess I need to do some build planning and see if I can come up with a build that has enough chance for heal procs and +regen to whipe out an AV without having to fit in aidself.
With an extropic chaos chance for heal in both T1 & T2 blast, a decent amount of +regen and added hp it might be possible.
With an achilles proc in Sleet and Infrigidate being able to stack, perma Sleet's normal debuff and spamming Sonic attacks, the amount of -resistance is crazy high with a Cold/Sonic
Sleet = 30%(+20% from Proc)
Infrigidate = +20% from proc
Stacked Sonic = 60% and up
Heatloss = 30%
You should be able to peak at around 160% -resistance but keep a consistant 110% assuming perma Sleet, 1 proc and Sonic blast
Add in assualt and some damage sets and the DPS starts to climb pretty quickly .
Looks like I know what my other build for my Cold/Sonic will be used for 
Achilles heel isn't as good as you think it is. The debuff it gives is seperate from the power effect. What I mean is that if you put it in sleet it doesn't give a 20% chance of making sleet a 50% debuff instead of 30%. There's a 20% chance that a seperate debuff that lasts for 10 seconds will go off.
The reason this is important is because I'm 90% sure that achilles heel procs can't stack from the same caster.
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The reason this is important is because I'm 90% sure that achilles heel procs can't stack from the same caster.
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Sleet is a different caster so the Achilles slotte din it can and will stack with the one slotted in Infrigidate and has the possibility, although very unlikely, to stack with a another proc from a second casting of Sleet
I know from farming myself in PvP that this is the case and keeping one proc up perma is very easy since Infrigidate can be spammed so quickly. I do realize that would slow down DPS, just saying
Benumb takes some pretty *extreme* recharge to keep perma, and it's even more difficult to reach the ranged defense softcap while still mantaining those levels of recharge.
Rad's are much easier to slot for both recharge (which has AM to complement, and it also provides end drain protection and mez resistance) and defense (even with the resists, rad infection still debuffs a good 7.5%, which equates into a hefty defense buff you won't have to IO for).
A mobile, non-interruptable self becomes invaluable in an AV/GM fight, and it also frees you a pool power choice (tough/weave) for not having to take aid self and that makes you alot more survivable.
Not to mention rad's debuffs are AoE (-regen, -dmg) which proves invaluable if you wanna be ballsy and solo multiple AVs at the same time 
Cold definitely has a bevvy of AV slaying capabilities, and their debuffing powers seem on-par with some of Rad's powers. You can definitely solo a number of AVs efficiently with a Cold/* def. However, (and I think it was Silverado who pointed this out to me), it would be really difficult for Cold to sustain the same levels of DPS as Rad, since its debuffs are much more fire/forget, and ice needs to re-apply debuffs every few seconds.
I think Cold may be able to eke some parity by having nearly capped -rech on the AV, which reduces incoming dmg by quite a bit, but that is inherently more fragile than Rad's style of AV slaying.
Take a Rad/Son & Cold/Son each comparably IOed. Solo some pylons (use insps, no temps, or bring a tank to just Taunt). Post numbers. 
An Offensive Guide to Ice Melee
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Take a Rad/Son & Cold/Son each comparably IOed. Solo some pylons (use insps, no temps, or bring a tank to just Taunt). Post numbers.
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Why would you need Insps?
The heal is the main reason to go for rad/sonic with AV soloing. It makes a huge difference. I have both a rad/sonic and a cold/sonic. My cold/sonic is miles better if I have any teammates, but solo the rad/sonic takes the cake.
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Take a Rad/Son & Cold/Son each comparably IOed. Solo some pylons (use insps, no temps, or bring a tank to just Taunt). Post numbers.
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Why would you need Insps?
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If you can do it w/o insps, go for it and post your times. Allowing for insps lowers the bar to entry. You need a pretty good build to solo pylons w/o insps or temps. W/medium purples, pretty much anyone should be able to. The point is to gauge how quickly a Rad & Cold can take down a mob w/massive HPs and high regen, which is why I also say you can just bring a tank to Taunt.
An Offensive Guide to Ice Melee
I know Rad/Sonic Defenders have downed a good bit of AVs in the solo game especially after the introduction of IOs to boost defenses/rech/etc. Well this weekend I started to look at what makes the Rad/Sonic such a good Defender for the job and I noticed something when I compaired it to Cold Domination.
Besides a self heal and Acc Metabolism, I didnt see anything rad does better. The -tohit portion from Rad Infection is resisted pretty heavily and since you can build for softcapped range defense with IOs that portion of Rad Infection isnt really needed anyways. So the main power that stands out is just the heal.
What I mean is this...
Rad gets Enervating Field for -resist (-30% toggle so its perma)
Cold gets Sleet for -resist(-30% long duraion and stackable for more than perma AND slotable with an Achilles proc)
EF also gives -damage but so does Benumb for Cold as well as all kinds of other effects to lower the AVs holds/debuffs/etc which are further dampended when used with PBU
Rad gets Ling Rad which is a very hefty -regen(the -runspeed is mostly useless for AV fights
Once again Cold has Benumb for -regen which can be perma with enough recharge in your build.
Rad Infection is -defense but so its Sleet AND Infrigidate
Add another point for perma end with Heatloss and another spell of -resistance to up the DPS. With my current Cold/Sonic the +recovery of heatloss is down for 10 seconds before its up again.
So besides a self heal and AM, and I do realize thata heal is a huge boon to AV fights, what does Rad/Sonic have over a Cold/Sonic for trying to solo AVs?