I have both diagnoses as well. The dead giveaway is that the attention problems are present regardless of the mood state. Stimulants don't make me manic, so clearly they do something my brain needs.

The other major difference is that the ADD is disabling in itself. Regardless of how the bipolar is going, ADD makes it hard for me to function (even drive!) without meds. Everyone has issues with attention deficits, but it's not a functional impairment for everyone.