If you want to feel strong and top the meters don't roll a DK.
If you're ok with bad mobility, subpar survivability and mediocre dps numbers then go for it.
UH dk isn't that good for m+ because we don't have any aoe stuns and our damage isn't that great. 90% of the time in m+ pugs I've been #1 in damage done but the only reason is that nobody sane would invite a dk unless they're overgeared so I'm mostly running with 845-850 ilvl groups.
I've done 7/7N and 1/7HC with my guild and been in the top 3 in all the encounters in a 25 man raid but some of my guildies are pretty bad so that isn't a good meter. Currently UH is sitting at the middle of the pack in warcraftlogs dps rankings but will probably be in the bottom 5 after next week because all the worst performing specs got buffed.
You should stick with your DH for now, they're in a great spot even after the nerf.
If you want to be strong, definitely do not roll dk. Both unholy and frost are mid bottom tier at the moment. Just look at the sims. Dks are literally in almost the same spot they were in WoD. Unholy especially. We did decent ST burst in WoD with breath, and it's the same now with apocalypse and soul reaper. After the initial pull though, our damage falls off big time when we reach the end of fights. Frost is worse even with the buffs. The sad thing is that we didn't get compensated for this at all, if anything unholy had its mobility and utility gutted (deaths advance passive, gore fiends etc). But it's not too late, scaling is on unholys side and I will definitely keep maining my dk.