Board Thread:Theories and Speculation/@comment-1512913-20150221101637/@comment-24071637-20150222061020

And Nagato doesn't have a Mangekyo Sharingan to use at all and Obito only ever got a Mangekyo Sharingan in his left eye socket until after his Rinnegan was taken. I suppose the fact that Ten-Tails/Kaguya/Shinju chakra makes it a kekkei mora and therefore Rinne Sharingan(like LegionZero said), meanwhile Sasuke's was born from his own chakra(as well as Indra's) and Hagoromo's. Thus staying at kekkei genkai level, therefore Rinnegan(with Sharingan abilities).