Air sealing is imperative. The connection between concrete foundations and wood framing is a place prone to air leaks and moisture problems. Wood is often warped, and concrete is rarely flat. There are at least three places for air to leak in and probably a lot more. Leaky connections can mean energy, moisture, comfort, and IAQ problems. Extending the wall sheathing past these connections is a good first step whenever practical. Caulks, adhesives, spray foam, and gaskets can seal them up tightly.
Unless you live in the desert, the ground is always wet; and that ground water is always pushing its way in. Footing drains can carry away bulk groundwater, but foundations also have to disrupt capillarity. Water in the soil will wick all the way up to the roof framing if you let it. Capillary breaks such as brush-on damp-proofing, sill sealer, and rigid insulation block this process. A foundation is a bad place to cut corners because problems are expensive and complicated to fix after a house is finished.
Not only do slabs eliminate the need for floor framing, but they also reduce the excavation and concrete needed for a crawl space or basement. And concrete slabs can be used as the finished floor. Monolithic slabs are great choices in any climate, but not any lot—relatively level lots are needed.
Insulated slabs improve comfort. Install a continuous layer of rigid foam insulation around the slab, except in hot climates or in termite-infested areas where local codes may prohibit the practice. The insulation buffers the slab from outdoor temperature swings, keeps the slab warm and dry, and lowers energy bills.