I think at the status-quo of the game, the best way is:
- go ahead with MSQ until you can unlock Potd
- use potd to level your main to 30, and the alt class required for job change to 15
- job change and again potd until 60 + kinna weapon
- back the MSQ and complete it, along with all the missed job quests to unlock skills

You need notice some "must-have" xclass skills require you level other classes as well. One good example is get swiftcast for healers. All potd-able, 'tho.

Note: leveling tanks, potd vs duties havent that great difference, but potd gives you the 255 weapon and this cannot be overlooked at.