Strength: Increases damage (and thus threat). Increases amount of damage avoided during block/parry.
Vitality: Increases maximum HP.

If you look only at the above, then STR and VIT both sound good. I'll ignore the others. However, you have to ask one crucial CRUCIAL question: "By how much?"

VIT is easier to calculate. It's something close to 14 HP per VIT for lvl 50 PLD. I might be a little off, but not much. 30*14=420 HP. A well geared PLD currently is near 6k HP. 420/(6000-420) = 7.5% HP increase from 30 VIT. Bigger % if your gear isn't so good yet.

STR is harder to calculate. But as you read around, the more common consensus is that 30 STR is pretty small effect. If you want agro, upgrade your weapon. Once you do, you won't have much agro problems.