If you have played DK for a while none of the specs is especially hard to play. All this "... lolunholyfaceroll" is starting to get a bit boring by now.
For me, both specs are fun. Unholy has its pet and are gcd intensive and frost has it big hits and insane burst aoe.
I play Unholy at the moment but it can change as soon as I get a new weapon as it tends to drop most mastery-items for us