Just to the muddy the waters, we use the U.S. edition because we think that the US edition is more challenging. IMHO, the standards edition dilutes what is so great about the Singapore curriculum because it is designed to cover all of the same topics that the typical US curriculum tries to cover. Although I'm sure that it is still far superior to most US texts, I preferred to avoid the "Mile Wide but Inch Deep" problem.

YMMV, of course.