IMO Fire because of reactive and scourge.
I think both sets have strengths... Fire is the better choice for just straight damage, it's quick and as Xan said, with Reactive and Scourge can be pretty lethal. Obviously it's not going to outdamage a Blaster or even a Defender before it reaches scourge HP, but Fire is probably your best bet for just straight damage.
Sonic has the -res component to it, which would help team mates attacking the target. You also have to consider that some of the Sonic animations root you for a long time (Shout, Screech).
I can't tell you for sure what you should do, I would also go with Fire Blast because I hate Sonic Blast, it just feels like a clunky, slow set.
@TheKatalyst
My **** is bleeding.
PvP community's thoughts on these two? Does one really edge out the other?