If you are comfortable tanking and know what you are doing you should go with STR main reason being that you do more dmg which helps keep aggro a lot better from BLM flares and WHM holy it also makes killing the trash in dungeons slightly faster, when you are a bit under geared or still haven't grasped tanking very well I suggest going VIT so if you do mistake you can still come out of it and also as stated above its good to go full VIT in the end content like turn 3 of coil PS.also little tip do the lv 40 job class quest for paladin you would not believe how many tanks I have seen go to stone vigil normal without having shield oath