It's really up to your playstyle and maybe the group you run with.

STR means enemies die faster, you hold hate better, and it makes your parries more "potent".

DEX means you parry more often.

VIT means you have more HP (which is even more magnified due to Defiance once you get Warrior). It allows your healer more breathing room. As an FYI, keep in mind that VIT only affects HP, it doesn't do anything for reducing damage.