Well, this still holds pretty true: A bad sin tank gets himself killed, while a bad jugg tank gets his group killed.
What do you want your challenge to be? Staying alive or holding aggro?
That was very true pre-2.0, a lot less so now. Guardian's got some great buffs in 2.0, especially with Saber Reflect (8k threat on everything in range, off the GCD plus whatever they throw at you reflected for more threat? yes please!)
At the moment, I don't think any of the tanks really shine in an off tank capacity, all 3 lose significant benefits when they aren't being hit and while Guardian Slash proccing Riposte helps Guardian's, you still feel quite Focus starved when not actively tanking. The heavily reduced regen for Shadows gives the same feeling when not tanking.
Utility wise, Shadow and Guardian are pretty even IMO. Shadow's Shelter is nice but not huge, the OOC Mezz is handy for mob skipping but rarely do you need to CC more than 2 things in a pull and Force Pull does make mob positioning a lot easier than Push. Guardian's get a bubble on their AoE taunt, huge mobility with 9s effective CD on Leap and Guardian Leap, AoE 6s Mezz and shorter CD on interrupt.
CDs I'd give to the Guardian. Saber Ward is better than Deflection, albeit slightly longer CD. Warding Call vs Battle Readiness is very debatable. Resilience and Saber Reflect both have their times to shine and personally, I love Enure.
Honestly, and please don't hate me for this, play whatever you enjoy more. I love the fast and furious play style of the Guardian while KBN and Kitru love the complexities of pushing the limits with Shadow tanking. As long as you are a competent player, both tanks will perform well for you and both will compliment a Vanguard co-tank.