Helicut gears makes it quieter. SQ also has slightly higher horsepower potential (bit more efficient). They are spec'd the same, but people tend to make more power with the SQ's.
I always have to try to hear a "sq" car if you know what I mean(Like turn an ear and be within 30 feet). S and T trim cars require no work to hear, in fact I always wanna hear more, the sq doesnt get me excited like that.