You should aim for 50% crit then try to focus on spell power. Same for stamina based dps. While magicka Nightblade is viable, you are honestly still better off as a stamina Nightblade. I mainly tank as Nightblade, but I have also done both kinds of dps as Nightblade. You will be able to do more damage as a stamina build, but more survivability as a magicka build because your siphon skills will heal your for quite a bit with all that spell power. You will definitely be taking pressure of your healer by jumping into a group of trash and spamming sap essence/funnel health. The magicka build is a little less damage/more utility vs. a stamina build imo.

Like I said I mainly tank, but tried both sides of dps. For example, there are 2 groups of Nereids in Cragolorn in the middle of a circular lake. As a tank I could not kill them fast enough before they killed me. As a stamina build I would take them down about half way before I died. As a magicka build I was able to kill 2 out of 3 of them. I attribute this to the higher powered heals I was giving myself while hurting them. Other people may get different results so take this information however you want. Either way good luck with your build.