Much of the answer will depend on the age of the AC unit itself as well as the history of repairs that have been done to the unit. Most AC units are designed to last 10-15 years even with regular maintenance, and the newest models are typically quite a bit more efficient. Unless the repairs are extensive, it is often better to repair a newer AC unit, whereas an older AC unit is usually better to replace rather than continue to deal with repairs that will continue to add up.