Good answer(s), Curiousity77!
I'm still half-asleep (yesterday didn't happen, or most of today either, so I'm in a real fog), but will still try to be coherent…

One of the biggest problems I see in discussing them, and one of the reasons people seem to have so much confusion about the difference is that there is so much focus on "mood", which is a word that can be understood in so many ways.

But one of the things I seldom see mentioned, and can be very helpful in helping to distinguish them is ENERGY. Mood, in a bipolar context is VERY much tied with energy level. Most people would say, "well, people feel peppier when they feel good", so what is your point? The point is that with BP, it's not just a mood shift, but an energy shift that is quite extreme and goes well beyond commonly experienced levels. For instance, when I'm hypomanic, I don't just feel that good sort of energetic level people would like to have to get things done. I get frenetic. Like a freight train on a zigzag course! There can be so much I don't know what to do with it. I could get the shopping done, clean the yard, and next thing you know, I've gone and cleaned the neighbor's garbage cans(!) And still be ready to jump out of my skin for not enough to do! The more I do I don't get tired (which would be normal!), I get MORE wired. To the point where I am spinning in circles. I will ...pick up a piece of fuzz on floor, heading to wash dishes, make it 3 steps, turn 180 degrees to fill a bird feeder, realize that step stool would look good in another color, run to get the paints, make another 180 suddenly remembering I meant to put some laundry in, oh shi*, there is an overdue library book, jump on my bike and pedal while standing up all the way there, get back and have no idea what I was supposed to be doing, so re-arrange the furniture in a room. Meanwhile finding little pieces of lint and running them one by one to the garbage lest I loose track of them again (because I can't keep track of anything for more than a nano-second). You get the picture. People will stay way clear, because it is like being in the path (no matter where you are) of a giant pinball (not to mention it's just plain scary to witness). That night you might well find me dancing around on the countertops cleaning out and re-arranging all the cabinets and singing at the top of my lungs. So, yes, it's an "up mood", but hardly the sort of thing people think of with the generic "sometimes I'm up, sometimes I'm down" discussion.

To be blunt, I've never seen quite that sort of thing in someone with BPD. (My BF has BPD, and while he will get more done when he is "up", it is nothing like this.) Hope that helps.