Whatever works. I found a great article in a magazine the other day about growing in bags of top soil. It was pretty efficient and the author said she had beefed up the soil each season, but the bags lasted her for years and years. She was up to something like 150 bags, LOL!
This is a similar acrticle:
http://www.rodale.com/growing-vegetables-bags
A while back, early in the summer, Lowes garden dept had two bags of soil with identical plants growing in each bag, to show the difference between the various brands of soil.
Of course the brand that they were selling had bigger plants :)