pve-wise, take all of /therm.
take howl and your first two blasts, then the stun if you can fit it.
pvp-wise you're gonna have space for 3-4 powers from your primary. it really depends on what you want to do. my sonic/therm went with sharks cause i've grown really fond of hibernate in team matches, so i didn't take the stun. if you don't go sharks, get the stun; if you get sharks, don't bother; let your team mez things.
I just made a sonic/therm mainly for pvp but pve alot also.. might be my new main... but can anyone give meadvice on pve an pvp for this build? should i roll a /rad instead? Does any1 have a good pve build? im not to worried about a pvp build yet. plz an ty