Central air conditioners are offered in 6000 Btu (one half ton) increments. Proper sizing is critical to function and humidity control. An undersized system will run longer than needed, decreasing your comfort. An oversized system (a more common problem) will "short-cycle" causing frequent on/off cycling. 

The shorter run times don't allow for proper dehumidification and will increase your energy costs due to frequent power "spikes", as well as significantly decreasing the system life. You will also want to compare the seer ratings of the systems you are considering. 

The higher the seer rating the lower your operating cost will be. As a general rule, each one point increase in the seer rating will reduce operating costs by about 10%. If you live in a humid climate, you should consider a variable speed indoor blower unit. 

This feature increases humidity removal and reduces operating sound levels dramatically. See the FAQ " Can you help me size an air conditioning system for my home? " for full details on system sizing.