It often comes down to how frequently you switch between the different environments. If you do so daily, for example, the easiest way is to keep the main catalogue on an external drive, or perhaps on Dropbox or similar service, and open it on whichever computer you're using.
But if it's less frequent, and client confidentiality is an issue (it sounds like this is your situation), then you're more likely to find that the best route is to open the main catalogue on the studio computer and then use File > Import from Another Catalog, which imports the catalogue from the field computer. This method (unlike reliance on xmp files) brings all the work into the main catalogue, and you can choose whether to move the images to the storage array or leave them on the external drive, if you're heading back to the client.