Just get what ever class is needed to unlock your job to 15. Then carry on through the story. It will gear you up and get you to 50 easily. If you own heavensward then all the lvl 50 quests and dungeons will reward you with xp and set you up perfectly for the expansion on your way to 60. Being ahead of that xp curve will mean almost no grind like those of us experienced at launch. You have the benefit of like a hundred quests of xp until you hit heavensward content, so if you like the game, don't hold out buying it.
The benefit of waiting to level up alt classes is that you receive and armory bonus. At lvl 50, all other sub 50 leveling classes will get a big xp bonus, that increases again if you have a 60 class. On top of rested xp, the blue part of the level bar, you will level up alts much faster if you just hold off.