A well built duct system doesn't leak and can be insulated. A well designed system will heat/cool the whole house evenly. Granted I wouldn't want to add a ducted system to an exsisting structure. The company I worked for custom built the majority of the ductwork on site out a cube truck. They did amazing work and it was reasonably priced. You can also use 1 AC unit and 1 furnace in a 3000 square foot home, of a medium quality brand and using exsisting ductwork, for about the price of 2 mini splits.