any of those will work. really all you need is a 3 foot hole with a little gravel in the bottom, plunk the 6x in there and then pack gravel real tight all around it. i built a 12x16 shed 21 years ago and just dug down a foot, layed a patio stone in the bottom and then dry stacked concrete block pillars and framed on top of that. still as level as the day i put it in. a lot depends on your soil composition too. my place is a good solid base of shale and clay about a foot down.
