If I get the chest piece, I'll probably never have to worry about expertise again and can change (probably) all of my expertise/stam gems for pure stamina. I can also use my Onyxia 25 helm (http://www.wowarmory.com/item-info.xml?i=49467) which give me more stamina plus a blue socket bonus. I would also be getting some bonus armor.
On the other hand, if I get the tier 10, I'll get the 10% damage reduction bonus which can further augment my mitigation rotation.
I'm not a particularly serious raider (I have a 10man guild still in TOC) and have only ran icc 10 a couple of times. I know that the Saurfang axe is pretty amazing for DK's and has a lot of expertise as well but I don't know when I will get that.