I would recommend caution when buying the cokes with a "sugar and/or HFCS" labels. These are NOT pure cane sugar, no matter what the lady at the store says. Only the ones that say "sugar" (no mention of HFCS) are 100% pure sugar.
The place I normally get Mexican Coke from (in Boston) often has a mix of the two versions, and I've tasted both. The ones that only say "sugar" are the ones with the sweet, clean, and refreshing flavors that the Mexican Coke is known for. As an added bonus, it doesn't leave a nasty film of HFCS in your mouth.
Of the ones with the "sugar and/or HFCS" labels, I've only had one that came anywhere close to being free of HFCS. Usually, though, these contain a sugar/HFCS ratio that seems halfway between the US and Mexican formula. Not quite as thick and syrupy as the US version, but not quite as light and refreshing as the Mexican version. This also means feeling only a slightly lighter film HFCSs afterwards.
The people at my source for Mexican Coke also told me that it was all 100% cane sugar. But, upon further investigation I found out that no one there bothered to read the labels, and they were simply repeating what they were being told. I had to find out the hard way by trial and error.