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This is incredibly subjective and changes by the update, if not month-to-month. If you are committed to playing a Nightblade, build one that is a Khajiit or Dark Elf. They are strong DPS races to begin with, and both are optimal for hybrid/flipping between magicka and stamina. In my personal opinion, the magblade is stronger because its easier to play in its current form. However, I run one with daggers on the front bar with a fire staff on the back, so its kind of a stamblade, too.
Agreed on Dark Elf magblade (with some hybrid skills). High Elf can also be top tier if you're sure you'll never switch to stam, but the Dark Elf gives that flexibility. Khajiit can be good for solo and 4 man content, but falls off in trials since the main bonus of crit damage will be negated by group buffs.
Magblade seems to have the better skills and morphs, with Merciless Resolve, Concealed Weapon, and Twisting Path. Technically the stamina Killer's Blade does more damage than Impale, but I prefer Impale's range. The main stamina skills you'll use are Barbed Trap, and maybe back bar a bow for Endless Hail or 2h for Stampede (slightly better than Wall of Fire).