Since both of these APIs are very similar and both provide the same GPIB functionality, which should you choose? If you are already familiar with NI-488.2 and are programming only GPIB devices, then there is not a strong reason for you to change to VISA. NI-488.2 is supported in all major application development environments, including LabVIEW and Measurement Studio. However, if you have instruments with more than one type of port or connection available to them, then using VISA might be advantageous because you can use the same API regardless of the connection medium.
Finally, many modern instrument drivers rely on VISA for their I/O needs, so if you are using instrument drivers, then you need to at least install NI-VISA for them to be able to execute.