Thunderclap is lower mag, it'll only stun minions alright. But it does give AOE containment, something an Illusion/Storm generally lacks. Of course until the 40s you've nothing really to take advantage of it...
Flash is better since i9 because you can throw 3 or 4 Acc/Recharge/Hold IOs & 2 recharges (or however many you need to get it to 95%) in and get better recharge from it. Its still long charging though. I have it and use it, but sparingly, generally when my PAs are down & I want to do a Freezing Rain & Ice Storm combo.
You get Tornado at 35 which has a (chaotic) stun in it too, same mag as Thunderclap. This means you could take that instead of Thunderclap. Or combine it to stun lieutenents. Course Tornado is quite situational, it can make an awful mess.
Thunderclap will also do well out of IOs, you could probably perma it with 3 or 4 Acc/Recharge/Stuns fairly handily, so you don't need 6 slots.
Really spectral terror is the AOE control of choice for you. After that I dunno. The idea of only stunning minions puts me off Thunderclap really, when I trigger a PBAOE with a squishy I don't want the Lieuts splatting me.
hmm yeah :/ will be getting tornado too (in fact the whole storm set if I can). Perhaps I'll take both Flash and thunder clap so I can actually see for myself and respec later if needs be. The long recharge on Flash is putting me off atm.
My next power pick is one of my not sure powers (everyhting else has been ooh I need this asap). Its currently a choice of mainly thunder clap/flash/o2 boost (maybe) or hover just to help my traveling (SS only..).
I'm used to playing a mind dom so when I play my controller I don't hold anyhting (unless Ir eally need to/dying/containment), I just keep everyhitng confused. So to I don't actually want Flash *gasp*. Looking at the duration/recharge on thunder clap and I could keep things "perma" stunned with 2 rech/2 dur (needs 2 acc too). Which is same slottign as I;d planned for flash really just hell of alot better on recharge. What puts me off now is the stun is only mag 2. I guess thats just enough to stun minions?
Chances are I'll take both powers really but can anyone with experience on both of these tell me if my idea (thunder clap instead of flash) is viable? All I know about thunder clap is it has a good animation :P.