Oh dear, there are so many options and so many different reasons for using each of them! I personally use ProMix but other swear by living soil, others create their own soil and others grow without soil at all - and I was just reading a diary where the fellow made his own organic soil and for some reason put kitty litter in it! I would start out doing as much research as you can on all the different options and then just simply choosing the one that seems to make the most sense to you and your conditions. My only advice would be to stay away from things like Miracle Grow potting mix or ANYTHING that "feeds" for x-number of months... you're going to want to control the nutrients yourself.
Good luck with your quest and Happy 420!