>>500052230Retail's the version that is guaranteed content updates for the next decade to come.
Classic you're basically left with a few options.
You've Vanilla Classic, which is just the vanilla classic experience with everyone already at the end game and currently won't receive any new updates
You've Hardcore Vanilla Classic, which is just well, Vanilla Classic with perma death.
Season of Discovery, which is Vanilla Classic's main content push currently and hasn't reached the end of it's patch cycle yet. It's the vanilla experience with some retail abilities brought back to Vanilla and trying to "solve" some of Vanilla's issues by adding more personal class content to chase and new quests and experiences that tie into retail's lore.
Finally you've got progression servers classic. Which went from Vanilla, TBC, Wrath, currently on Cata with Mists of Pandaria soon to be announced and expected to release next year.
It's one continuous version of the game that just progresses from all the old updates until they choose to stop.
Ultimately what you want to do, comes down to you. All versions of World of Wacraft share the same sub and you can essentially play them all as you feel, aside from the latest wow expansion which you need to buy to access on retail. But can engage with any of the content before War Within on your subscription and currently has a 30th anniversary for the franchise celebration event in game, that's planned to last till January.
But bare in mind, Retail, Progression Classic and Vanilla Classic/SoD all have separate clients despite sharing the same sub
