I asked on the ptr and this guy said that frost was to burst down bad players as 2H(dunno about DW) while Unholy was harder to play but more rewarding and does better in pvp(dunno about 2h or DW)

last I played dks was frost in 4.1(imo deathknights at their best since 3.0) and im I have no idea how they are now.

If possible I'd prefer 2H unholy but back in cata unholy just really wasn't cutting it vs frost