Why two story or story and a half.

One of the biggest expenses you will have in building a house is the foundation.  Building 600 square feet on the ground is quite a bit of foundation.  Poured foundation costs would be through the roof and probably more than I would spend on the entire house.  Cutting that figure almost in half is a giant savings.  If you are doing posts with a concrete footings, it cuts down dramatically on the time you will spend with a Post Hole Digger or a shovel, and on the number of bags of concrete you will have to mix.
Also, with a second story, you get to use your heat twice.  In the old days it was not considered necessary to pipe heat up to the second floor, just a grate in the floor works very well as heat rises.  I think that we should revive this practice.
As stated earlier, it is better to cut down on the square footage of building skin exposed to the elements,  A story and a half, with only a 3 or 4 foot upper side wall cuts down dramatically on the exposed skin of the building.  The roof is half the size of one on a sprawling building and heat is conserved.