The difference is that not all devices could accesses using a serial connection.
Example: The Bluegiga Blutooth Adapter is seriell, so both solutions may work, but a Logilink Bluetooth adapter ist not a serial adapter so the ser2net/socat solution will not work.
Another example ist that with the usbip solution you may link a USB Memory stick over the net or other usb devices like USb Soundcards etc.
If the ser2net/socat solution ist fine for you do not heasitate to use it.
Hope this helps