Depending on the software provider for the air card, you may find yourself needing to complete an authentication protocol by inputting login credentials into a third-party software application. The software application is usually provided by the 3G (or now, more commonly 4G) provider. If the air card is simply integrated into Windows, with authentication based strictly on the device's registration with the provider, you will have a more precarious situation in prioritizing network resources the way you want.
In most circumstances, you can simply unplug the air card when you do not want to use it. Therefore, you will simply be using the Cat5 network. This is the most common way of utilizing the portability inherent to removable network devices. When you do not want to use the on-board local area connection, you can simply disable it in Windows.
You can disable network adapters in Windows by going to Start -> Search -> View Network Connections
Usually, your CAT5-based ethernet connection can be disabled and enabled as the "Local Area Network" network interface card. This will be unique based on your hardware, but is commonly found to be a on-board Realtek or Intel NIC.
This is one way to segment the use of both devices without using overtly complex methods. Simply enabling and disabling the devices, or removing their physical connections, will allow you to separate and prioritize their use quickly and efficiently.
When two network interface cards are configured in Windows, irregardless of if they are WiFi, 3G/4G Air, or Wired connections, the default behavior is for Windows to attempt to use the connection which responds the fastest. However, this can be offset by a number of factors including the time it takes for the connection to reach certain domain name servers. It is possible to prioritize the use of multiple network interface cards using the manual "Interface metric" setting under Advanced TCP/IP settings for the individual adapters and using multiple default gateways. This may involve using a router whereas you can segment traffic and provide multiple default gateways. This requires extensive configuration, may not be compatible with the air card at all because the hardware may bypass your router altogether, and is an overly complex solution unless you are doing this in an environment with hundreds of users swapping around hardware.
My suggestion would be to simply disable whichever connection you do not want to use at the time. Again, the best way to do this in Window 7 is simply to disable the Local Area Connection (wired NIC) in Network Adapters. Thus, you will not need to physically disconnect the Cat5, but can prevent its use while the Air card is online.
If this suggestion does not work for you, please let me know.