Nightblades aren't built, they are drawn into creation from the blood of the guilty and suffering of others. Lol. You raise a couple of issues. Nightblade is my favorite class, and I have several trillion hours playing them.

One, stam or magic? Of any class, I feel that stam and magblades play the most distinctly from one another. Pre-Morrowind, magblades were hurting. However, the update 14 changes have really helped. Before in pvp I'd shrug off magblades. Now they can murder you...there's a PS4 D.C. Magblade xx-mojo something or other...man that toon hits like a truck. Anyway, now both are viable for end game, in my opinion (although there are still areas needing improvement, but there always are).

Stamblades are playing like brutal assassins right now. Lots of weapon skills, lots of ganking, a fast and furious rotation. Magblades feel a bit more like dark magicians...a bit more aoe and buffs, a little less dps in exchange for stealth. They're both fun to play...some people strongly prefer one or the other. I mightnrecommend rolling both and playing them to like level 20 (if you have slots). It takes a bit longer to see the distinctions because stam and magic builds not pull away from each other at as low a level as other classes, but you start to get an idea how they play in their teens.

Second, race. I am not a fan of obsessive players who demand a race be used. Honestly, you can pick any race and easily approach 98% of the game's content. You could even be an orc stamblade. But let's assume you want to be viable in end game, and narrow down the choices.

For stamblades, conventional wisdom is Khajiit, bosmer or redguard. Khajiit and bosmer are very similar. Their stealth passives are identical. Their recovery passives offer 21% stam for bosmer and 20% health and 10% stam for Khajiit. The Champion system will even them out a bit, but this one is slightly advantage Bosmer. The third is 8% weapon crit Khajiit, and 6% max stam plus poison resistance for Bosmer. This one is slight advantage bosmer. Honestly, the races are so close you won't notice much of a difference exceptmin extremely specific circumstances. Redguard s return stam in melee fighting, and are also a decent choice. For magblades, I think alter are considered best, but dunmer and bretons work well too.

For endgame, either stam or magic is viable in pvp and Pve. In pvp, either lends litself to ganking, although that's a bit of an oversimplification. Stealth guerilla warfare has many applications...I annoy and distract as much as I gank. In pve, I think conventional wisdom is that magblades work a little better...but I prefer my stamblade. If magblades do perform better, they only do so slightly,,and it would only be owing to their super fast ultimate regen and Eye of the Storm, the destro ultimate. But I think Eye is OP, and I wouldn't start a new character assuming it'll be this powerful in the future. I sense a nerf on the horizon...but maybe not.

Hope this helps. Have fun murdering things.