I've found Obliterate hits a lot harder on low armor targets.
It destroys cloth more so than say a plate target.
Obliterate, after Outbreak to apply both diseases to a target, is strong DPS... and it combines exceptionally well with Hungering Cold to CC most targets while you burn one, possibly with Empower Runic Weapon too.

The Howling Blast is usually a better choice.
It is AoE, and glyphed applies Frost Fever.
Which talented reduces movement speed, essentially snaring anyone hit to the same level other classes would snare us.
So we're hitting multiple targets for some damage and the kill target for a bit more, snaring multiple opponents.
Plus, you can hit an opponent at 30 yards, while Obliterate is melee range only.

Use both tools, when you want to burn someone...
Obliterate puts out more single target DPS, on a lot of targets.

Here's Multibock's thread for DK DPS: http://www.dual-boxing.com/showthread.php?t=43219