Unholy is better for significant reason beyond mobility and better necrotics.
You get Unholy frenzy which can break partners out of ccs and is a substantial dps cd for other classes (lock for example), you get a petstun and pet damage while cc'd but it requires some management. Anti-magic zone is great for yourself and teammates and your green shell is better. You bring 8% more magic damage debuff which is again great for running with casters.
If you do go frost, mostly what I see is 2hander oblit spec, sometime I see 1 handers with no oblit.
BTW get 240 spell pen for mages with mage armor.
I dont have a DK but this is what ive heard/seen from 2200+ partners.
well if u use obliterate u cant use necrotic since they both use unholy runes,and if u spec into obliterate ure pretty much going to have to sacrifice lots of other points if u want to get imp strangulate or desecration,and if u dont go obliterate spec then dual wielding becomes redundant since the reason u go dw is especially to make oblit hit with both weapons,if ure oblit spec keeping blood plague up is kind of mandatory since it boosts obliterate damage and if ure not i think it still does more damage than a necro,unholy presence 100%of the time,and get 195 spell pen since mages will probably go with frost armor against you,stack mastery especially if ure necro/hb spec,
blade barrier is probably better then bladed armor especially if ure the train target
I started out frost until I got comfortable with the class. Learning to play unholy now, it's so much better! I just miss the burst.
it's not obvious burst but it's still burst.
You stack necrotics up much faster than frost dks too so when you hit them with a huge pet+gargoyle they're massively fucked unless they've been keeping up with healing.
Lower rated healers won't and I came out of an arena at about 1900 mmr where the priest did a penance and flash heal at like 10% hp and neither heals even moved his hp bar and my pets finished him off while i was stuck in a cyclone ;p
Its all comp dependent. Frost can arguably be better in Triple DPS, TSG and possibly DK/Rogue/healer otherwise UH has a clear edge and even in those 3 UH will work as well or nearly as well. I personally think UH is a better spec with ANY comp when played well. UH has probably one of the top 3 skillcaps so it isn't easy to squeeze everything out of it unless you actually know wtf your doing. I used to be a huge Frost proponent but not after I got back to playing UH and remembered how much better it is.
People that aren't good enough or just can't play UH well enough think Frost is better and for them it is because of how simple it is but it almost always is a L2P issue. But the reason why I hope I don't have to play Frost again is that the only difference between a 1800 Frost DK and a 2200 one are his partners and to me that's disgusting and terrible spec design.
Also go 2H its more survivable even though its less burst (yes it is less burst due to the ToT mechanics and RI) due to DW being too RP starved to use LB and Sac on demand. Also as DW you HAVE to spam HB for DRunes just to come close to enough RP to sustain FS's and that is bad because of how strong NS is. Basically NEVER use Oblit unless they are wearing cloth/leather and are sub 25% health WITH KM UP because a non-crit Oblit hits for less than a NS at twice the rune cost.