Based on GSM Arena's review, the American version is louder, since no EU law to comply to. Their tests suggest the signal attributes are slightly better on the International version, but not something the ears will likely discern.
Frankly, the sound quality seems amazing and the extra volume in the American version is nice for pushing higher end earbuds that usually have higher impedance. Let us hope evil Apple does not persecute consumers further and the SG3 avoids legal holds.