I recently started doing RBGs, currently ~1900 MMR.
I somehow prefer playing unholy to frost - I really like the numbers with diseases popped at the right time and overall pressure I can do.
Would switching to Frost help my team? Can a well played Frost DK do even more damage? Or it's maybe less damage but more useful dmg - since eg. obliterates hit 1 target much harder so easier to get a kill?
I think a well played frost DK can do more damage than unholy, but unholy is still very viable in RBGs. The major thing I dislike about unholy in rbgs is that you have to be in the group to do your aoe pressure, which can leave you over extended and in risk of dying, where as with frost you can howling blast from afar and still do absurd damage, as well as switching from aoe pressure to single target burst on a whim when going for that kill where with unholy you have to set it up.
Unholy, well played, has the highest pressure a melee dps can put out. Necro's are great for kill targets, disease spread pressure is ridiculous and your pet offers extra burst and cc. I've seen a good UH dk pump 30+ million a game, which on top of a boomkin, spriest, affliction lock and destro lock oomed their healers in no time.
imo, it depends on your team. if you'r team got alot of dot classes, unholy would seem to be the better choice, as you would all be multi dotting. but your dots alone with just bursters around you would not make a huge difference afaik.
I actually run both. Unholy has some tools that let you extend a little more (mainly being able to sit in BP and not incur the damage penalty that frost has), and the spread pressure is actually higher than frost. Unholy has single target pressure as well, it is just not obliterate numbers, it is rely on your team numbers. As frost sometimes I can score solo kills on offtargets, but as unholy you have to necro stack your team target. The reason I play both is because my main job in node games is to spin the flag with howling blast. I can do that from range, while unholy requires you to be in melee to spin a flag (blood boil). The maps where there are not nodes (flag maps, mines) the pressure unholy brings is better.