If I can chime in on this - Ronald is right. The only difference we ever see with the second (client) pc on the network versions is a slight lag time compared to using the server.
The functionality of v14 is really the same as with v13, I'd say - no major changes in how anything is done by the cashiers. A number of really nice features are added, and some functions (like the product export and import) are labelled a little differently but still used the same way as in v13.
Also, it's easier than you might think to set up the network version. If you have any kind of network between pc's now, there's almost nothing to it. If you don't have a network, it's not too bad to create a network with Windows (hey, even I did it!).
As an alternative to Ronald's example of running the client in your office, I added it as a second/backup cash register. Besides the pc, it's just the cost of the cash drawer, really, since the receipts can be networked to the printer at the server. Even if you have just one clerk on, you can be doing your work nearby while your other register is churning through the transactions. Or you could be tinkering with the server while the cashier is using the client.
Hopefully I'm not violating any license agreements, but I have a non-connected version in my office. I use it as a "test" system to try various changes and the resulting effects before exposing the data in my active version to them. If the data in my test system is getting a little old, I just copy & paste the EZP folder from the server (through the pc network) to bring it entirely up to date (e.g., if a trial change went really bad and I want it out).
I can still update inventory an all that from my office computer by using one of those remote programs to look in on the active registers. If a cashier is using one of them, I'll just switch to the other. It works out very well.
That might be just the thing to get you to switch to v14!