Someone just asked me this question, today. As a dentist, I’ve always said it really doesn’t matter, as long as you’re doing both (concentrating at the gumline, of course). It’s like asking whether it’s better to wash your arms or legs, first, since flossing cleans between the teeth, while brushing cleans the sides. But I’ve never really checked it out with an authoritative source. Here’s what the ADA has to say about it:
“Either way is acceptable as long as you do a thorough job. However, if you use dental floss before you brush, the fluoride from the toothpaste has a better chance of reaching between teeth. Some people brush their teeth and unfortunately skip flossing because they think their mouth feels clean or they may be short on time or tired and flossing is postponed. That’s not a good idea.”
So, there is at least one reason why it might be better to floss first. But the most important thing is that you are doing both!