if you go to SE93 transaction - the transaction to define transactions - then you can see that there is no difference between ME21N and MEPO, except the transaction name itself.
ME21N is listed in the menu, MEPO not, if you search for them using transaction search_sap_menu.
From my knowledge MEPO is the basic new design (as of 46 release) transaction to create POs.
it allows create, change, display in one single transaction, while ME21N, ME22N and ME23N have a dedicated entry screen create, change, display. you can even switch to the other options too, but only if you have authority.
And this is as well the main reason for ME2xN transactions, they are used to segregate authorization.
