We use the US edition. We don't use the CA version I prefer to go with the original version. I believe that the US edition is the same like the original Singapore one with the exception of some of the spelling/names and units. The US edition uses US currency and adds US units (gallons, yards, ...)
DS6 uses the workbooks and Challenging Word Problems, which I find excellent as well as essential. At one point I bought two teacher manuals but never used them. We also have all the textbooks which I use as a reference, if that.
I buy directly from the Singapore Math distributor
http://www.singaporemath.com/Primary_Mathematics_US_Ed_s/39.htm
