Most of the time you will get better genetics, you can look for seedbanks in the posts to get an idea of which ones to use, but generally Serious Seeds are extremely good for the price and highly recommended, T.H. Seeds also comes to mind. There are a few others but I can't think of them off the top of my head.
In case you are wondering if genetics make that much difference they will, if you get a pack of serious seeds all the plants will look and grow the same and have the high, that is what you are paying for. If you get nirvana for example, you will get all different phenotypes so the plants can grow and look different while giving you varying degrees of highs. In a nirvana pack you will probably get one good plant out of a ten pack where serious seeds will have all ten be good plants.