Done CORRECTLY the zoning is the only way to achieve control of up and downstairs. You can zone with a damper system or you can zone with multiple units. The issues of noise and reliabilty all hinge on the quality of the product and contractor chosen. One thing if you choose a damper system is you should use a dual capacity outdoor unit(A/C) Running the indoor fan(s) 24/7 with raise the humidity in your home during the cooling season. So what do you have the appetite for and/or you willing to compromise comfort and control for the up front savings. If you are concerned about repair/ownership cost buy a FACTORY extended warranty that will cover the complete system.(equipment and zoning) The decision making matrix will consist of several items but only you can bring your priorities to the forefront. First cost, level of control, warranty, energy savings, brand, reputation, and ..........................................

We often condenm that which we do not understand.