Juggurnaut Tanks still make better tanks than the Jedi Guardian's do at least at the lower levels. My level 13 Jug can take on an elite 2 levels above it. While my level 16 Guardian Tank cant even take down a elite at the same level as she is, and I have all the best gear for that level on it. Plus the opponent is melee based which the Guardian should be strong against. So while the Jug Tank is not as good as the Assassin Tank (Which my main character is by the way). It still has its uses and can be very effective if used by the right player. All the Jug tanks I have teamed up with have out lasted everyone in my groups except for maybe myself, and thats pretty good from where I am standing.
This has nothing to do with the differences in Guardian and Juggernaut as they're essentially the same, and everything to do with the companion each gets. Vette is about 35% better at damage dealing than the Droid and has a better selection for gear, making her an overall better companion.
In quest/solo play the Jugg has a clear advantage early on. But in 4 person flashpoints they both play the same and perform the same.