Now I think you only need the $295 ATEM mini if you attach it as a web camera to stream your edit live, but if you want your edit to be live on YouTube or to record it you could go for the mini pro.
Still the Web Presenter may in fact be a better solution for streaming for me. Since I have the $495 Blackmagic Ultrastudio 4k, I have spare SDI ports and that way I could stream a 4k sequence as the ATEM MINI only accepts up to a 1080 60p sequence while the Web Presenter accepts up UHD 60p sequences though streams out at only 1080 60p. Of course I lose the ability to see me in a picture and picture via a gopro.