It's all about the genetics. Sure, there are exceptions but as a rule, the better the genetics the better the plant. You can't make chicken salad out of chicken shit. The better the genetics the more consistant your plants will be. If you buy a pack of Serious seeds and grow them out, you will usually get 2 types of phenos. If you buy a cheap seed like Nirvana and grow out a pack you are liable to get 10 different phenos. With Serious seeds, you will always get a pheno that is "mother quality". With Nirvana, you will be lucky if you get a "mother quality". Again, there are exceptions to this rule but once you have your enviroment dialed in, genetics is the most important aspect to your garden.