My HVAC friend said as long as I stayed with one of the top 4 or 5 companies I would be happy. Most of the companies have several brands.
I bought the complete Carrier 3 ton 18 SEER Infinity Series with furnace. It works as a system with a high tech thermostat that controls everything. 5 Speed heat pump so it only goes as hard as it needs and a variable speed air handler blower that you normally don't even hear running unless it is very hot or very cold. The outdoor unit in a lot quieter than my old 13 SEER one stage unit, on the lower speeds it is about as noise as a 20" box fan, even on the highest stage you can have a conversation while standing next to it.
They make an even more efficient unit but for the TN climate, my research didn't look like it would save enough to have a pay back. I asked the salesman and he said the only reason to buy the highest efficient was bragging rights to my friends.