Well, this is how it is...
Blizz had the brilliant idea of introducing easy-to-get epics. They get more and more epic every few months... But older gear isn't updated.
So as of Season 3, it's hard to see why you'd use Heavy Clefthoof over Vengeful Gladiator. (I haven't checked all pieces, just the chest piece)
If you take a look at the chest pieces only:
HC Armor: 500
VG Armor: 529
HC Stam: 45
VG Stam: 54
HC Def Rating: 24 -> About 10 Def Skill, which amounts to about a 0.4% chance not to be crit'd.
VG Resil: 26 -> About 0.66% chance not to be crit'd.
Both have 3 sockets, though admittedly off the top of my head I can't tell if the colors make much of a difference for a tanking Druid. Also, the socket bonus of 4 Dodge Rating is easily overthrown by the massive stats on VG. (Str, Agi, Int thrown in for good measure) You also get Hit Rating which is always nice, and Crit Rating to help with threat and rage generation. And some armor ignoring... which I really don't know what it amounts to.
Now, with all that being said, Clefthoof is definitely a cheaper alternative and it is easier to get as you don't even have to be LW yourself, while the VG will require a few weeks of Arena.
Also, that is only for the S3 gear. The gap between Clefthoof and S2 is smaller, and S1 is somewhat similar but I'd say inferior to Clefthoof.
So basically stick with Clefthoof until you can get S3, in my opinion. If you don't like/want to Arena, just stick with the PvE upgrades. Keep in mind, however, that the Clefthoof is only 3 pieces and for the other pieces, Arena gear is often a nice alternative.