I have taken into consideration both approaches and here is my take on this highly debated topic.
If you floss first, you're dealing with a larger amount of plaque. You'll see a lot more plaque being removed than if you floss after brushing. Flossing gets the plaque out from under the gums and in between teeth. But I also see flossing as 'fine-tuning' after getting the big stuff out of the way with the brush. Once the big load is removed, the fine-tuning can be more effective.
In conclusion, I floss after I brush with the Sonicare, and rinse thoroughly.
Happy flossing!