No idea. So not much help, but I do know that my sith juggernaut spec into vengance does more DPS than my friend's immortal juggernaut. If you want to be a heavy armor tank, then you spec juggernaut. I tend to fill the off-tank role, so specing vengence works well for me. I'm squishier than an immortal, but there's good stuff like Unstoppable, Ruin, and Impale that come in handy. Smash is really nice with vengance: less cooldown, more damage, no rage; great for clearing trash as an off-tank. Immortal is the way to go for main tank. That is why people really want the dual-spec toggle option.