Ive played both private servers and retail servers retail is by far a better experiance because in World of warcraft the whole game is 1000x better as a multiplayer game rather then small amounts of people on a small private server,
Private server dosent actually have to be private all it means is its privately ownd and hosted rather then Blizzard owning the server, it can also be altered to do things that you normally wouldnt do in the standard retail the problem with that being that there are ofturn lots and lots of bugs when the private server is changed to do something differnent to the standard wow,
I recommend if you make a private server that you make a Server that is for PvP mainly, where you start at max level and have vendors that allow you to purchase standard gear say season 10 gear, and then make it so that when you kill a enemy in pvp you get gold and use that gold to buy the next seasons gear and so on,
and i 100% believe that if you have donator gear that you be very carefull not to make it to over powered ive played alot of private servers that make donators very very overpowered and it just ruins it for everyone, id say about a 7% stat bonus over the best gear that normal players can get would be surffice. allowing skillto be able to beat donator gear if the person without donator has more skill then the donator.
TLDR Retails > private, Private = free with more bugs, retail = pay with no hassel
PM if you wanna talk about it more Cptwin.
PS. ive played wow for around 6 years with a couple of breaks inbetween i stopped around feb this year