Gas and Oil Home Heating Furnaces - Dual or single units?

I've seen advertising that seemed to tout dual unit systems for two story homes as a feature. However, tonight I found out that single unit systems are used on homes up to 2750 sf. Is this more efficient? Is it more cost-effective?

I have heard many horror stories about two story homes that wouldn't maintain consistent temperatures upstairs and down in NC. Are newer, single unit systems with electronically controlled dampers able to overcome this? Are they more efficient?

Its all in the way a hvac looks at it Id say. With me its look at the whole lay out of the blue print. Where ,what and how you can do the duct work. Most of the time we will put in more than one unit. That way like for the upstairs say big AC and smaller heater. For the downstairs part of the home. Bigger heater and small AC. Zone controls work but this way if one unit does go out on you the other unit will still work some. The units are closer to where they have to put out the heat or cool. Most of the time the blower cost is less if it goes out with the smaller units and then you dont have the zone controls that can go out on you. Its just How you look at it. One unit for a 2 story home and no zone set up. The builder just went the cheep cheep way;) ED