Currently unholy is the best dps spec for dk, but for that you need the new trinkets evil eye and tail tip. Frost is great to but around 540ilvl with trinkets unholy beat frost.
So unholy is the winner
I don't think the trinkets are needed for UH. The reason dw frost pulled ahead early 5.4 was due to the trinkets, but the buffs to frost and UH made UH stronger as a whole. Even the Klaxxi trinket is very viable for UH.
Your take away from this is both are viable. Unless you are pushing world first (9 months late for that you are) they are within a few % of each other. Your skill with a particular spec will have more of an outcome than the spec balance. Play what you like the best.
OP says "just returned". No point telling him go unholy as it overtakes frost at higher ilvl, unless he's got a way of getting to that kind of ilvl in a couple of months. Either way, if you're starting fresh again, frost will probably be more rewarding early on.
Not arguing that unholy is eventually better, because it is, just trying to answer best as per what OP asked for.