If you're in a hurry to join end game content that requires a rank 50 class, just get THM to rank 50.

If you have a static party, you probably just want to stick with one job to keep pace with everyone else.

If you're not in a hurry, you may as well rank both at the same time to make it more convenient to get leves.

Use wands and a shield to get your shield rank to 50.

VIT is important for HP (especially if you're soloing)

MND is important for MP (especially if you're soloing)

PIE is important for DoT damage (but this doesn't really matter until end-game if you're doing a DoT-centric DPS mage)

INT is marginally important for nukes. Kinda depends on if you're fighting mobs that are not that close to your level (usuall party grinding and leve mobs) or much higher than your level (bosses and NMs). This should be easy enough for you to test yourself. Vary the amounts of INT you have and see if it makes a difference on the enemies you usually fight.