Ultimately this question can only be answered by discussing your particular situation with a qualified AC contractor that you trust. There are general considerations that will help guide your decision conversation. You must consider the age of your system(s), how often your system runs, and where you live. Remember that living here in coastal Chesapeake means can be quite corrosive to outside systems. Generally, if your system is ten years or newer and you don't have a major system failure, you should consider repairing your existing system. If ten years or older and you have a major component failure you might be better off replacing the central air conditioner in Chesapeake, VA rather than repairing it. As they say, it just depends.
Simple things like blown fan motors, capacitors, and other low relatively low cost items can be replaced. However, if you things like a refrigerant leak, it can be costly simply to hunt down the source of the leak. Then you have to fix it. Compressors of course are a costly component that rarely should be replaced once it is out of warranty. Old systems that use the now federally phased out R-22 refrigerant more than likely should be replaced in favor of the more efficient and environmentally friendly R410A refrigerant.
Give us a call. We'll make every effort to stretch out the life of your existing systems. Ultimately you have to decide but we'll give you all that you need to know to conduct your own cost/benefit analysis for repair or replace.