You asked, Sensi Seeds or Nirvana? That's like asking, Mercedes Benz or Kia?
Sensi Seeds owns White Label Seeds. I would not bet my nutsack that the genetics are exactly the same, but the closest thing you will find to any Sensi Seeds strain is the White Label version, that is of course if White Label also offers it.
In this case White Label does, but only in feminized seeds. You can purchase 5 seeds for $80.98 or 10 seeds for $150.87.
While if there is the option I always pick regular seeds over feminized , in this case if I were you, I would go with the feminized White Label.
I would grow from bag-seed before I would grow any Nirvana strain.
Additional: As was previously mentioned, Sensi Seeds Skunk #1 is a tremendous value. You will never find an easier strain to grow, it yields very well and it is still a quality strain.
$44.45 will get you 10 regular seeds or $31.75 will get you 5 feminized seeds or $60.34 will get you 10 feminized seeds.
(From Attitude)