Which is better, flushing solution or regular tap water in an eyewash station?

There are advantages and disadvantages to both. Most flushing solutions are made with saline that is similar to human tears, typically the same pH. They also contain preservatives to help reduce the growth of bacteria. If maintained properly, these flushing solutions are perfect but there is a limit to the amount that can be stored in a unit. Tap water is unlimited but it can contain chlorine and other chemicals that can cause further irritation to the eyes. Choosing which one to use will ultimately be determined by location and resources.