I depends on what you want to play, be it SCH or WHM, as you can't start AST until you reach level 50 and advance the MSQ enough to get to Ishgard.

SCH has a benefit in that you can run as a SMN for leveling and as a SCH for dungeons, but ARC is a strict DPS for the first 30 levels so you won't be able to actually queue as a healer until after 30. Past that point being able to swap to SMN makes leveling easier, though SMN is also very spammy in how it works prior to level 50 so it doesn't really help much in breaking the monotony of Ruin/Stone spam.

CNJ-WHM is straight healer all the way through, though this means for field and MSQ you will be mostly spamming stone.

Once you unlock it you can switch to AST if you prefer, and they as a job start at level 30. They also play identical to WHMs for most of the leveling process.