It depends.

Whenever you're being attacked, do you prefer healing yourself or letting others heal you?

+ STR will increase your damage output. This affects the enmity levels you are able to reach and the amount of self-healing you can do. It also contributes a little to your group's kill speed.
+ VIT will increase your overall health pool. This affects the amount of spike damage you can take and the amount other players can heal you for.

Stacking VIT is a viable option for both PLDs and WARs. Because a Higher Health Pool obviously means that they can take bigger hits from enemies.
However because of (i) their self-healing abilities and (ii) their regularly-used weaponskill cycles pulling less enmity than a PLD at endgame, stacking STR is also a viable option for a WAR.