Is it best to build a home 18 in. or so off the ground or on a concrete slab.?

The advantage of a crawlspace, which you describe in your second option, is that it's easier to run mechanicals. This includes HVAC, plumbing, and electrical. You can potentially change the layout of your home if you want to down the road. Building on a slab requires that everything be laid out exactly where it needs, and will not allow you to rearrange a kitchen, move a bathroom, etc., without extensive work. Foundation walls are required anyway, at depths of 2-4 feet below grade depending on what your frost line is, so you might as well at least build on a crawlspace. Hope this helps.