While leveling, if you want to tank, you're better off just using Defense, since that's where a vast majority of your threat and survivability comes from, especially since AoE is way more important while leveling and Defense just does it better.
Once you've gotten to 50 and started doing Ops, switch to the hybrid if you feel like it. It's got slightly better mitigation (~25 hp/sec in absorb shield and ~2% better K/E mitigation (the difference between 4% absorb with 50% shield and 4% K/E DR)) and a good deal better ST threat and damage, but worse AoE threat and damage, not to mention that it's got a *much* tighter attack string and resource management.
is the standard Defense spec you'll want to use (there is some minor variation as to what to do with the last 3 pts I invested in the Focus tree), and this
is the "standard" hybrid spec (there is a lot of variation in how it's done, the important parts are getting Overhead Slash and Commanding Awe in Vigilance and Blade Barrier in the Defense tree, along with all of the major mitigation talents that you get coming up through Defense).