Having tried both(Or the republic mirrors at least), I definitely suggest Assassin.
I've played my Shadow as a dps for a long time, and damage was fine, but "fine damage" isn't worth being so squishy.
So I respecced to Kinetic Combat(Tank spec) and have been enjoying it since.
You have really really good damage for a tank, really good survivability, awesome self heals, really good utility with lots of slows, stuns, aoe knockback and force pull. High mobility with Force Speed(This one is my favorite, and I wouldn't trade it for anything else.).
My Juggernaut is not half as fun nor powerful.