The skill level between frost and unholy is pretty small. I know a lot of DKs like to swing their ***** thinking they are better because they play unholy, but in reality they are both pretty easy specs.
Feral's have a higher skill cap overall to play at the top level.
It is up to you what you want to play...if you are just starting out I would suggest a DK. Feral is definitely going to take a lot longer for you to pick up. If you want a bit more of a challenge then go feral.
I personally would just play both and just see which I would enjoy more.