Personally, I prefer multiboxing Retail since it's faster paced and there are more QoL features in the game, whereas I thoroughly enjoy Classic from the solo perspective. While you can wait for Shadowlands if wanting to try out Retail, I'd argue that the end of an expansion is the best time to jump in since all the catch-up mechanics are in place and the economy is established, full of items that have leveled out in price and are readily available.