In my opinion...

go both but consider only for...

Tank for the causal dungeon contents so you can farm the tomestones faster.

DPS for the Raids so you can focus more on the boss and mechanics rather than boss, mechanics, team, your own HP, defense reduction time, and etc.


Tank Job is very heavy on responsibility when it comes to keeping yourself a live and trying to make things easier for the entire party but casual content should not have much issue with how you tank as long the group understands there should not be much of a big deal with casual contents since with small pulls healers have more room to DPS with the group and big pulls put Healer more focus on healing and less DPSing.