Right now i'm using the 251 warrior helm, mainly to activate my 2p bonus. Tonight, finally, the Broken Ram skull helm dropped, and I picked it up. So obviously, its a higher item level, so my its going to have more stam and armor. I lose some parry, but gain dodge and hit.
my question is whats the threat value of the 2p bonus, and is it high enough to keep my 251 helm over the 264 offset?
Extra damage is always nice but you gotta look it this way: do you want more survivability or more threat/damage? And also, do you need more threat in the first place plus it's completely dependant on the fight you are doing. Festergut? As a tank, screw the 2p bonus and go fo maximum survival, three-inhale hurts and your healers will need every bit of help you can give to them. Lady Deathwhisper? Maybe go for more threat-orientated set and T10-2p nicely boosts our snap aggro tools even further.
Long story short: gear (and spec & glyph) for the encounter. Still, in any kind of progression content you always want to go for maximum survival - wear your best EH pieces.
Broken Ram Skull is way better choice than 251 T10 piece.
I recently got my 2pc and I do love getting 11-12k crits on bosses again. For fights with tighter enrages such as blood queen for my guild I like to maximize my dps as much as possible so it is a nice boost. But like the previous poster said, for hard hitting progression fights you want to go with whatever is gonna make you easier to heal first. If it's bosses that you have no trouble with or that don't hit hard, then I would consider sacrificing small survival for more tps / dps.
All in all it's ~32 more stam, 104 armor, and 74 hit (2.24%) vs 24 str and 2pc bonus. Id say bank the t10 until you upgrade to 264 2pc because the increase hit is going to go almost as far for tps unless your at or near the cap.
Long story short: gear (and spec & glyph) for the encounter.
Use an addon like itemrack to build multiple sets, have a set which includes the 2pc bonus and is geared around threat. Use it for fights like deathwhisper, dreamwalker, maybe saurfang... fights where snap agro (or smooth tank transitions in the case of saurfang) are more important than absolutely maxing your survivability.
When you get a token and upgrade the tier helm, don't throw the rams head. I use mine with a frost resist enchant and a spell dmg meta at sindragosa.