Nobody should be passed over for this by merit of role in the raid. We're talking about a weapon that will be literally months of farming ICC in the making. You aren't trashing your one dps or your one tank who will or will not have it months from now by giving it to the best raider (nevermind the roles of tank vs. dps), period.
If your guild is a huge success in ICC and you cream hardmodes early on you might have 2 (maybe 3 if you're farming hard all the way up to cata release). By that point we're talkin about farm content. No raid needs 2 of these things to be relavent, efficient, or effective on a dps or a dk tank.
Bottom line: the discussion of who is benefits more in terms of raid effective role is moot, by the time your raid has one, you'll have put most of ICC on farm anyhow (or you'll be struggling the same amount on hardmodes you were struggling with before).
It's orange text, it's about prestige, dedication, and nothing more. Any guild who is giving this out to someone other than their most valuable and dedicated player (out of those who can use it) is making a mistake. Be that a tank dk or a warrior, paladin, dk dps, it doesn't really matter.