Titans are obviously a mixed bag in the current version.
The TEC loyalist Titan (the Athena Class pictured above in the header) is nigh indestructible, as its abilities focus on survivability. Further, it can drastically reduce damage to other ships in your fleet as it levels up. None of the other Titans are nearly as dangerous, as they can't preserve your entire fleet. I smell a need for a tweak, or everyone will be TEC loyalists.
Worth noting is that the Vasari Titans kind of suck, but that's probably why you can't play them yet. If you know your way around Sins, you'll notice that the ships are actually in the Rebellion Beta, but you have to do some programming tweaks to access them. They are on par with the Advent Titans, but again, the TEC loyalist Titan is far and away the best of the bunch.