I'm not a very big fan of Sonic Resonance, so unfortunately I don't have any good advice for you. Illusion is a pretty powerful set with any secondary tho. You didn't mention your APP or alpha slotting but for this pairing I'd be thinking about Fire and (probably) Spiritual.
But you also really lose nothing from walking away from a character. In the end we all end up walking away from them. They are just experiences we've had. I'd say put her in the cooler if you're really just not enjoying the play, and don't delete her until you really need the space. If you need the space now and have major heartburn about deleting her, you can always pay to transfer her to another server.
while i am somewhat fond of the set(i have one shelved in 30s myself), sonic is not too popular for several reasons
-high end costs
-terrible for min/maxing and sets bonuses
-its resistance...and game mechanics are in favor of defense
-its signature power, liquefy, has an ungodly long recharge and low duration
ill/sonic is a fun combo for teams, but the 2 sets just dont have synergy for soloing imo beyond sonic siphon to add to your ST attack chain.
i also agree with fire mastery and spiritual alpha.
@Injenius~Virtue
Stonefather - 50 Stone/WP/Soul brute
Sable Affliction - Earth/Dark/Fire Dom
Wild Cipher - Beast/Time MM
I'm pretty fond of Illusion, and feels that it goes well with every secondary. So far, I have Ill/Rad, Ill/Storm, Ill/TA and Ill/Cold all at 50, and I'm working on a second Ill/Rad and an Ill/Kin. I might try an Ill/Therm someday, but I have very little interest in pairing Illusion with Sonic, FF or Empathy. I have a Mind/FF, and have no interest in doing another */FF character. Sonic has some positives with Illusion, in that the Resistance Debuffs will help Phantom Army do more damage . . . but the couple of times I have tried to play a Sonic character, I just find the set to be uninspiring. It is kind of too bad, as there are plenty of pun and joke names that can be created based on Sonic/sound/noise, and that's an important part of a character for me.
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
Playing a Grav/Sonic at the moment, and I will say that Sonic is definitely not going to 'wow' anyone. Its effect on gameplay is low-key and not really overpowering. If that's what you're looking for, I would suggest trying something else. I like Sonic, but it's not really a top tier set (hell, it's probably the bottom tier). I've never played Illusion so I can't really help with that.
Grav/Sonic does have some nice synergy I've found, particularly between Wormhole and Disruption Field, although I'm mostly using it as a concept for a 'gravity' secondary.
They ALL float down here. When you're down here with us, you'll float too!
@Starflier
I have a level 50 Illusion/Sonic that I am fond enough of that I've created a "decent" build for her that provides her with 43% ranged defense. The big problem with sonic is that most of it's effects are fairly subtle; adding 40% resistance to every teammate doesn't have the same kind of feeling that /rad has. The other problem is that it is an endurance nightmare. I have the Cardiac Alpha and a Miracle proc to offset the end drain, but it is still very high.
I view Illusions/Sonic as a big moving bunker. Everyone who stays close to you can benefit from increased survivability and an army of pets and pseudo-pets to take aggro.
Yeah End cost is very high from what I recall and damage is lowlowlow... and I have to say I've never really noticed the extra resistance...
I think I might consign her to history.
Thelonious Monk
Have a mind/sonic that took a long time to get a handle on, especially to solo her. Have also made a Ill/sonic build for a friend. While sonic doesn't make a team 'godly' like FF can if fills in alot of the final gaps in strength nicely.
I guess you make a fair point. I've got a few other Illusion Trollers and they seem to be far superior even at low level. I don't need to decide her fate immediately because I've got plenty of slots left but it does seem remarkably under powered
Thelonious Monk
I have an Ill/Sonic who went with sonic entirely for concept, despite my misgivings. He's still in the DO levels, so it's too soon to say. He's in a "hardcore" supergroup, meaning that if he is defeated he gets the axe.
Last night everyone else on the team (all hardcore as well) died and got deleted, but my guy lived (barely). Since I don't have Sonic Dispersion yet, I lived only because of Phantom Army and fast footwork. Note that everyone else was wearing my sonic shields at the time they died. 
My hope has been that (especially after I get SOs and Dispersion) the resistance bonus would slow down the process of defeat, giving the team more time to react tactically or reach for the inspiration tray. Didn't work too well last night.
If we are to die, let us die like men. -- Patrick Cleburne
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The rule is that they must be loved. --Jayne Fynes-Clinton, Death of an Abandoned Dog
That's because people aren't used to working with Resistance buffs, only Defense buffs. There is definitely differences in how you have to play since you're taking steady (though a lot less) damage instead of only being hit now and again.
I just rolled a Ill/sonic... wanted to make one smiler to the Ill/kin on a different server... and I like it so far, but it also fits the concept better
I have an Ill/Sonic at 50. It's not much fun to play because it seems very under powered but I'm relucant to delete it because I rolled her on the 5th Anniversary just before Positron's special address. But as she goes, she feels really weak - makes my Earth/Thermal seem overpowered by comparison and at least he has some great mitigation to compensate for lack of damage.
I like Illusion as a primary but I do feel from my (albeit limited experience thus far) that it needs a secondary that either has a very noticeable effect, and Sonic seems to lack that.
Perhaps I'm wrong but I've never felt it was a hugely successful pairing and frankly I cannot remember ever seeing another, so maybe there's a reason for that
Thelonious Monk