Either way if done well would work however, for long term use of this building as a living quarters, i would advise the piers. Main reason being is of course settlement, with the piers you would be able to use jacks to make any adjustments to the level of the building, it will also allow you to pumb thru the floor at a later time vs. tearing up the slab to put in a drain line or whatever. As far as critters go, I'd rather not be at their level "on the ground" also at a later time if you wanted to enclose it you could make good use of some of that timber you're cutting, all you need is a portable sawmill and there ya go. Once again, as I said earlier this would make for a better long term plan. :-)