Honestly, any spec will work well, I've noticed DK's have a great capacity for damage dealing even in a tanking spec or semi-tank spec.
More the question you want to ask is what does your raid need more support on, what sort of tanks do you have? If you have very strong aoe tanks (Unholy DK, Prot Pally) then you're probably better suited with a stronger single target threat build. If you have some ace prot warrior(s) or bear druids, then an aoe-centric build would be better.
At the moment, the more I crunch the new patch state of affairs the more I get the sense that there are 4 very general build styles I can come up with (though nuances range significantly for specific small talent placements here and there):
1.) AoE heavy Unholy (focus on spell damage, UB, CE, etc)
2.) Single Target Unholy (focus on Necrosis, BcB, ScS, and supporting talents)
3.) Deep Blood (can tank 1-3 target very well, but is no better than bear or warrior on aoe)
4.) Frost Balance (decent aoe with HB spam, stronger single target)
These are general and there is plenty of hybrid in between. Frost and Unholy offer the best spell damage tanking tools (though blood isn't far behind at all), though each tree offers some strength. Blood now offers some very reliable single target life-span/survival tools, very rugged. Frost offers the best hybrid threat style, while Unholy is stronger at one or the other if you specialize.
So, first question: what sort of group comp are you hoping to OT for?