Nirvana's strains are considered to be newbie strains. The reasoning behind this is that they are cheap and cheap for a reason. Nirvana doesnt hold any parent stock and most of their stock is f2 or worse. What this means to you is that the strains are unstable, you will see variation across the grow and you may only get 50% of your female plants that show the traits as advertised on the site.
Cali-high mentioned a top seedbank, that is the flying dutchman, also mandala seeds are pretty good and neither are that expensive, also you get decent information on the strains and 7/10 they are an f1 hybrid, which will give you better more consistent results.
I've been checking out sensi seeds a bit recently too and I'm looking at some skunk #1 seeds, super skunk and sensi star.