have you considered two seperate systems?
even though zoning would be less expensive, most builders around here(cheap) always install two(cheap) systems in a home such as yours. in this scenario you should have at least one working when the other fails, and they will fail.
The only way I would consider a two system approach would be when the selected A/C system needs more than 5 ton to meet the house sens heat and latent heat gain. Other than that, I stick with zoning.
And, you really shouldn't have a problem with the equipment when installed right.
In my line of business, the majority, over 85%, of problems are cause by the installation, not manufacturer's components.