The A level books tend to have more arithmetic and the B level tends to have more geometry. I'd say start with the A level. Rather than starting on Lesson 1, skip to the first review. If she does all the review correctly, move on to the second review. Hit all the reviews until you find where she should begin with her lessons.
I personally prefer the standards edition over the US edition. The "US edition" is identical to the Singapore books except the font has changed and the Singapore money has been replaced with US dollars. The "Standards" edition is divided up a little differently to better correspond with the American school year, has Americanized names in the word problems, and includes topics such as probability and statistics that are taught in American elementary schools but not Singapore elementary schools.
