until at some point there was only the openpli. than some teams decide to make something of their own and drive the development of software with a different way.(so i think there aren't big changes between them). how many and what are these differences i do not know and i can not say, most of them aren't visible. maybe hardware support,maybe something else. both of them are open,and the support is daily. the openvix is a member of o.e alliance.
you can find both teams open code in their git.
as a conclusion i would like to say that i tried all of them (and a few which do not support xtrend any more) in my et9000 and i found all of them very good. the reason for that is the receivers kernel (3.8.7) which helps in support. so the image selection is done at most with what is more practical for everyone.
personaly i am using now satdreamgr image.its a pli based + many tools and plugins which i find useful.