There's been a lot of interest as of late in the rather new, super high efficiency AC and heat systems manufactured by manufacturers including, Carrier, Goodman, Trane, Nordyne and Lennox. While each of these companies provides what they call, ultra high efficiency units, only two of them actually break the 20 SEER level; Carrier and Nordyne.
SEER, or Seasonal Energy Efficiency Rating is a measurement of the economical efficiency of an air conditioning system, or how little energy it uses to keep your home or office at a designated temperature setting. The higher the SEER level, the lower the utility bills should be. The ultra high efficiency units by Nordyne and Carrier are able to almost cut your electric bill in half (depending on other ways you use electricity).
What most people are going to care about when they want to install a unit like this is, "How much does it cost?", and, "Can the cost be offset by the lower utility bill inside a few years?". When you think about one of these systems, you are easily looking at $10,000 for a 3 ton system. Of course the price depends of the amount of work that needs to be done to install it, and on how much of a margin the company installing it wants to make off of a unit like that. Some companies in San Antonio will easily charge upward of $15,000 for Carrier's Infinity Series or Nordyne's IQ Drive, which can be found under brand names such as Frigidaire, Westinghouse, Tappan and Maytag.
Now, which unit will probably cost you more money, and which is more economical? Well, since you actually are paying for a name, the Carrier Infinity Series will be the most expensive system you can find. Of course, like I said, that depends on the contractor doing the install as well. The Nordyne unit boasts up to a 24.5 SEER unit, while the Carrier unit attains a 21 SEER maximum. While neither unit is anything to sneeze at, you may ultimately want to compare the warranties between the two systems. I haven't been able to prove that the Nordyne system actually has more than 21 SEER, but they say they do! Your best bet is to ask your local HVAC contractor if they can get either unit for you, and then compare them.
Make sure when you DO compare the units, you are comparing price, economy and warranty. You must compare all three to understand the full scope of what you are having installed in your house.