In my opinion, as a complete starter, go retail.
There are a few reasons but the main reason here is that classic Servers in any form are way to grindy to enjoy the game fast as you dont have knowledge.
Depends on what you enjoy on a game tho. Playing alot of competetive PvE or PvP, you will have more fun in Retail.
If you are seeking more of the communicative plays with other players and enjoy the journey of the World of Warcraft through the leveling progress, you may be better on with classic.