I was a rogue in BC, and then in WOTLK I mained a Rogue, but alted a Mage and Pally for raids. In my eyes it has always been Rogue vs Mage in terms of what to main, and it’s tricky. Rogue has a lot of social fun regarding being stealthed, and enables lots of situational combat choices that no other class really has (Except Feral Druid) for PvP, and working your way up the DPS meter against other melee is a fun goal in raids. Mage has satisfying AoE, and a ton of conveniences though, which includes a reduced gear dependency compared to something like Rogue, making leveling easier.
I’ll default to mage as the main as well for Vanilla, but Rogue and maybe Warrior will be PvP Twink alts.