Should I play WoW or WoW Classic?

Hey! I’ve just started playing WoW for the first time and tried the free trial up to level 20, I really enjoyed it! Now I’m thinking about getting the montly subscription, but I’m a bit confused about which option is best and what the differences are between Retail or Classic WoW.
I’d really appreciate it if someone could help clear things up. I just don’t want to level up my character on the wrong plan!