The flip organic insers are squares of organic cotton that you tri-fold before placing them in the wrap. The stay dry inserts are microfibre with a 'stay dry' top layer to keep baby comfortable. Hope that helps!
I have both inserts in my stash and the organic inserts will hold more wee than the microfibre ones. But the microfibre ones are still good and work well for my 11 month old. She is a medium wetter though. I could never have used the microfibre inserts with my older DD as they simply weren't absorbent enough for her. I guess if depends how much your baby wees!