Rad blast is one of the weaker Defender sets, and sonic can be quite a slow if not coupled with a higher damage secondary.
Sonic is really a glorified Force Field, it has some decent debuffs, though, so they do make the set have it's own flavour.
Rad trollers are, obviously, quite different from /Rad blast defenders. Rad blast has it's plus points, it has quick attacks that have decent debuffs with them, however it lacks any really depth and is best coupled with Rad primary, in my opinion.
Sonic/ shouldn't be getting mezzed by anything other than sleep attacks (meaning Freakshow, Carnie Illusionists and Rikti Mesmerists, mainly) due to Sonic Dispersion... 
Sonic's main benefits over Forcefield are Clarity and the two -resist debuffs. Liquefy is also situationally either pants or superb. Sonic also offers a lot of personal +resistance to stack with the Defender Epic shields and/or "Tough" from the fighting pool - it's quite possible to hit the Defender S/L resistance cap.
Sonic gameplay really boils down to running Sonic Dispersion constantly; Clarity on the Empath (premptively on others if you can be bothered or there are sleep mezzers about), ST -resist debuff for bosses, AoE Anchored -resist debuff on a teammate always (I often find that adopting a scrapper works better than a tanker), Bubble everyone every 4 minutes and use Liquefy if the tank gets into trouble (the -defence and -tohit are great, the knockdown is pants). Sonic Cage is rubbish in PvE apart from against the Arachnos Flyer and Certain EB confrontations (LGTF!).
Personally as a Sonic/Rad I'd either go for the Dark or Psi Epics - Dark for the +damage buff and good all-round resistance, Psi for Dominate - (a high damage, low activation time hold and is spammable!) and the shield to fill your only remaining resistance "hole".
For comparison, I quite enjoyed my Sonic/Elec's ride to 50 (Elec being even less damaging than Rad!) and he even saw his share of soloing!! It does take a bit of creative slotting and the willingness to play "team support" more often than other Defenders though. I took Fitness, Leaping, Medicine (Aid Other/Aid Self) and Leadership (Assault/Tactics) and was usually quite busy on teams.
Forgot to add - keybinds make life much easier if you're getting fed up with apllying bubbles and buffs.
I have "Target Teammate" bound on Numpad1 to Numpad8 on all my toons, my Sonic Defender uses "/" to toggle_on the ally-allied debuff AoE, with "-" and "+" for the two Bubbles, Clarity on "." and Aid Other on Numpad0. Tapping the right Ctrl button toggles on all his personal toggles (shields, leadership etc) sequentally one by one using XX_toggle_on$$YY_toggle_on.
All my main attacks are still on the bottommost tray and bound to the standard 1-0 keys, but Liquefy and Sonic Cage are bound to the numpad (* and Enter respectively) as they're more "supporty"... 
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Getting really fed up constantly spaming my 2 shields and having to re spam the one on the tank every time i get mezzed and everytime i fire off my rad AOE's i become a squihy agro magnet 
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Spank the tank for being poo or wait a mo so the tank consolidates attention before firing them off. It is I think better if you use the AoEs. Either that or your too close to tank sharing aoes which you don't always have to be.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
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Thing is i love my Rad trollers , got 3 of em and thought it be similar on a Defender but seems not to be
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If you want that set, you need the Radiation primary - which happens to work very well with the Sonic Blast secondary. The Rad secondary, as people have said above, is a bit lacklustre.
That said, I know people who do honestly enjoy both the Sonic shield set and Rad Blast. I actually have a Corr with the Rad Blast set as her primary at 50, it's by no means unplayable.
Irradiate is quite a good debuff as well as doing AoE damage, but yes, it will tend to get you killed earlier on in the game. I found as I levelled further I used it more, as other people tended to have all the aggro. I'd think that on a hero team with a Tank, it could be even safer. You just have to watch when you use it. The other AoE is really quite weak I think - takes longer to cast that Irradiate for less damage and less debuff.
As Sonic you shouldn't really be getting mezzed too much thanks to Sonic Dispersion - unless you've been fighting a lot that use sleeps recently? Those are annoying on Sonic and FF, but at least not that many mobs use them.
Overall, I think because Rad Blast does have quite a few forgettable powers, if you're going to take it, best to do it with a primary that will keep you busy. Sonic and FF are largely shield, turn toggles on, forget for a few minutes until it's time to shield again or if the toggles drop - so it's a good idea to back them up with a secondary that's useful or that you find fun. For instance, my Corruptor with Rad Blast is a Thermal, and she tends to be kept pretty busy by that. Also, I have all of her primary, so it doesn't bother me that there aren't many good powers to take in Rad Blast.
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As Sonic you shouldn't really be getting mezzed too much thanks to Sonic Dispersion - unless you've been fighting a lot that use sleeps recently? Those are annoying on Sonic and FF, but at least not that many mobs use them.
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While playing a FF defender it felt like all mobs that do mez uses sleep. As you can't feel other effects muchly only Sleeps and KBs gets your attention.
Been slogging through my Sonic/Rad Defender (at least it feels like a slog) to lvl 31 and just wondering does it get more...anything...really with my remaining primarry/secondery powers still to come?
Getting really fed up constantly spaming my 2 shields and having to re spam the one on the tank every time i get mezzed and everytime i fire off my rad AOE's i become a squihy agro magnet
I've tried FF twice to mid 20's and gave up but trying to stick with the Sonic Shields longer , just thinking Rad as my secondery may have been a mistake?
Thing is i love my Rad trollers , got 3 of em and thought it be similar on a Defender but seems not to be
Any words of encouragement appreciated from Rady Defenders out there