StamDK is in a better place than MagDK for PVE, at least if you're interested in competitive end game raids. StamDKs have the potential to parse the highest DPS numbers. MagDK can parse high, but take a melee spot where Stam characters do more damage. Stam's heavy attacks and MagDK's Flame Lash both consume off balance but since Stam does more damage there isn't a place for MagDKs for groups going for leaderboard scores. MagDK also needs to proc Power Lash to sustain since their abilities cost so much. StamDK also has an easier rotation.

That being said I still play my MagDK in end game content and progressing on vet raids with him with my guild.

For solo stuff, MagDK is easier to play due to good self heals and access to the light armor shield. I can solo Skyreach on him but have issues trying the same on StamDK due to lack of shields.

Both are DOT builds, with StamDK focusing on poison damage and MagDK focusing on flame damage.

For PVP, both are kind of bottom of the rung at the moment, with MagDK probably being in a bit better place. MagDK does good in duels but relies on block casting for defense. Both are slow and have a hard time finishing kills fast and their DOTs just get purged. I've PVPed with both, but not the most experienced PVPer so someone else could chime in. The devs seem to wanna push DKs into being tanks...

MagDK Dunmer is best due to the increased flame damage, and flame resistance means you can go vampire for the extra sustain and not be hurt too bad.

StamDK Redguard and Khajiit, with Redguard having better sustain and Khajiit having more crit. You heavy attack in your rotation for dual wield so sustaining isn't a huge issue, at least in my experience.