OpenEmu has been in development for a really long time, first around 2009 which is before RetroArch came around. Our focus has always been a native OS X application while theirs has been cross-platform. Also, cross-platform UI's are a pain!
One has a polished cocoa UI, the other is a cross-platform app. There's no animosity between the two projects here in case anyone is curious.
OSS with polished UI's are rare and desperately needed. Nice work.